I'm here, sorry it took so long. So far, very very good advice from the group! I am more than impressed, and encourage you to take their words to heart.
It does indeed sound like a bacterial infection, though a picture would help to further diagnose this properly.
A few important notes for you about keeping a betta.
1. bettas are solitary fish, and do best alone. They are extemely aggressive, especially with other bettas, and that would explain his having eaten his tankmates. I'm surprised Petco didn't at least warn you about that.
2. bettas are quite temperature sensitive, and while some climates will allow for no heater in their tank, most still require it just to keep it steady. Bettas thrive most in temps of 80 - 83, but the important thing is that the temp be steady. Without a heater, unless the room they are in is very warm year round, temp controlled, it's not possible to keep their temp steady. Room temps will fluctuate with day & night changes, even in southern CA. I lived in Barstow for a few years, so I understand the climate in southern CA, and yes... there can be significant temp changes from day to night even there. Constant and/or rapid temp fluctuations will weaken a fish's immune system, which leaves them prone to illness.
3. There should be no need for any type of air stone in a betta tank. Betta's are air breathers, so they need to be able to reach the surface to breathe. They belong to a group of fishes called labyrinth fish. The labyrinth gland is what allows them to breathe air instead of pulling water through the gills to get oxygen. Most bettas prefer a very still environment. The circulation from an air stone can cause stress, which also weakens the immune system.
4. Water changes are extremely important. Bettas are strong fish, but there is still a limit to what they can withstand. The smaller the container the more often water changes need to be done. In 1 gallon or less, without filter, the water should be changed at least twice/wk, though it is much safer to do at least a partial change every other day.
5. While this is a great group of people here on the forum, I don't always agree with everything that is said. The required feeding of twice/day is not something most bettas need unless they are fry (babies). Once/day is plenty, whatever he can finish within 1 - 2 minutes. A betta has a very small stomach, and over feeding will cause a number of problems, though I've not known constipation to be one of them. Polluted water is the worst part of over feeding, but a fish needs time to digest what he has eaten. Feeding once/day and a water change every other day will ensure that he has time to digest his food properly and will keep the waste levels down to a minumum.
6. Water testing is very important, but the good part about a betta is that they tolerate 100% water changes very well... provided the changes are done frequent enough so that water params are never "bad".
7. Most bettas also don't tolerate a filter running, again... a stress issue. If working with a small enough filter, preferably something you can control the flow on, then it can be known to work, and this will reduce the need for such frequent water changes.
In your situation, I see a number of things that have probably played into the problem. The lack of water changes is the #1, temp fluctuations would be #2. Dirty water will expose your fish to bacterial and fungal issues, and temp fluctuations will make him more vulnerable to both of those things, as will water that isn't warm enough.
I really don't like to give out medication suggestions until we are positive of what we're treating for, but in your case I am going to suggest a combination of methylene blue and fungus eliminator. If he is in a plastic/acrylic tank, it would be a good idea to move him to a glass bowl for the 10 days of treatment, or upgrade him to a small glass tank of 5 gallons or less.
Get a bucket, mix the meds into a bucket of water, dosing both according to directions on the bottles. Methylene blue is 1 drop per gallon, fungus eliminator is 1 tsp for each 5 gallons. The easiest way to dose this is using a 5 - 6 gallon bucket. Measure the water going in, please be sure not to overdose these medications. Mix meds into bucket of water and then let sit for 10 minutes, then mix again to be sure everything is thoroughly dissolved and mixed. Use the medicated water to do a 100% water change daily. Once mixed the water is good for 48 hrs, or 2 changes, then must be remixed fresh.
Continue this treatment for a full 10 days, even if your fish appears to be doing better before then.
These types of bacterial infections can be difficult to get rid of, so I can't promise you a 100% success rate. It will depend on how strong your fish is, how serious and advanced the infection is, and age. Average life span for a betta is 3 - 5 yrs, so he is getting up there in age already. I have seen some very sick fish recover from these infections, but that was when all criteria listed above for their conditions was met.
The meds I suggested will be useful in the future should you attempt to keep another betta. This is a great combination of preventive meds that are potent and still safe for a betta. If he's not sick, they won't hurt him, will simply give his immune system a boost. If he is sick, these 2 medications together will treat most of the bacterial and fungal issues bettas are most prone to. Standard treatment is 10 days, but longer treatments can sometimes be needed. Hang onto those meds once you get them, every betta owner should have those 2 things on hand. Any new betta coming home from the store should spend the first 10 days on that treatment.
With all that said, I also have to mention one more thing before I go. The deal with constipation has arisen again, as I read a suggestion to feed your betta peas to relieve it. Please
don't do this! Bettas are carinvores, which means they eat meat. Their pellet foods are made of primarily meaty ingredients to ensure they have good nutrition. The digestive tract of a betta is not designed to handle heavy vegetable matter, such as peas. While the fish may eat the peas, that does not make it good for him. Babies eat candy, that doesn't make it healthy. I don't know who started the whole idea of feeding a betta peas to relieve constipation, but I strongly warn everyone against it. Peas act as a laxative. Imagine making castor oil a regular part of your child's diet... or running for it everytime your child fails to poop on schedule. All that does is mess up the stomach and digestive tract, and the end result is an early and very painful death in the fish.
It sounds to me like you got some very poor help at Petco, which doesn't surprise me. Most of those people don't know squat about fish, and the few who actually do, most of those know very little. You have gotten some good advice here thus far, I hope it's enough to teach you what Petco should
have taught you from the start.
Best of luck to you and Petal!
If you need more help or have further questions, please ask.