Go to the thread about my betta where I said he is acting differently - because my betta stopped eating after I had him for almost 2 months. There is a lot of information there for you to look at - I tried almost all of it.
By the thermometer (sp?) to check the water - it's cheap and it's worth it to have it if you want your betta to be happy.
Mine loves the pellets - I thought he didn't like them anymore but he's eating again and happy - he just needed a small resting period. What I did was put one pellet on a spoon with water and let it get softened and then put it gently on his water. He started eating right away. Remember they do not need a lot of food - so one pellet in the morning is fine. Plus that pellet expands with the water so... you should not feed him anything bigger than the size of his eye.
The flakes are harder for my little one because they are too large. You have to break them up for them.
I even tried freeze-dried bloodworms. Again, he ignored these. And really these are just a 'treat' - it's not a main food thing.
Hopefully your betta is just getting use to his new environment. Water must be between 75 - 80 degrees. Changing water, the same way - anything colder will make a betta sluggish.
I have two plastic trees in the acquarium I bought for my little one - sometimes after a water change, he'll go hide behind them because he's mad at the disruption. About a day or two later or when he sees his food coming - he gets right back into the groove of things.