Hmm, could have been from previous exposure to ammonia. Albino fish also sometimes have red gills, did he come this way or develop it?
Best guess if it's not those two things is some sort of infection, perhaps he's damaged himself some way or irritated his gills and they have become inflamed. Problem is, I wouldn't know what would cause it. I had a fish with one side that way because another fish had slammed into her and attacked, that was the only damage she got. I had used an antibacterial(little salt and some melafix) and she fixed up after a week or so, the inflammation and redness went down faster. Next best guess, but you should probably find out everything before doing any treatment.
What exactly ARE your water parameters? And what test kit do you use? A lot of people think they are where they should be to find out they're actually not. PH swings, or wrong PH can cause itchiness and they can bang themselves on stuff and cause damage.
And how big is the tank?
How many fish and what kind?
If you want better answers, we do need some more info, otherwise it's all guess-work..although sometimes even with all the info there may not be any full answers, but you have a much better chance of it.