He looks just like my beta (other than perhaps being sick and all). Since it's winter (especially in Maine I'm sure) - I would pick up a 50 watt heater for his tank and keep it on 78-80 degrees. Technically they aren't suppose to need heaters but they don't naturally live in Maine either. Just looking at him he looks fine. Are there places he can rest in his tank - for example, I have a couple of pieces of driftwood and my beta just lays on that driftwood in great comfort - he doesn't have to swim all the time. Have you declorinated his water before adding him? Have you done any water changes lately - I know they are only one fish and they don't need water changes very often but every month or so would likely be helpful. Are their any live plants in his environment to add oxygen to the water?
We tend to treat our bettas differently because they are only one fish. For example, bettas tend to hate a lot of water movement - swimming against a current is difficult for them so often they don't get provided with filters. I still think you need a small filter to aerate the water, just like you would for any other fiish - you just don't need gushing streams of water. Same with the heater - they don't need a high temperature to live in except in places like Maine or Kansas (we had a blizzard yesterday- I've lost my stray cats since they haven't been back to eat). We also don't think we need to change the water because the amount of ammonia coming from the poop of one fish isn't very high - but they DO poop and if they accummulate a bunch of snail buddies so do they. They are kits you can buy to look at all the major water parameters but you may not need that yet.
Pick up a small filer (Aqua 100 are a good size with slow current). Pick up a small heater. Make sure there are places to rest in the tank. Change the water - making sure you add a declorinator like Prime or Tetrus. Tetrus Plus also has the added advantage of providing much needed bacteria to the tank. Most people don't bother to cycle Beta tanks - and that may be the problem here - there isn't enough bacteria in the water and the ammonia and nitrites have built up to toxic levels.
Finally, Betta's, like Gourami and a few other fish species breath regular air using their swim bladder which helps lower and raise them to the top of the tank Swim bladders give some species problems and quit working and they die - the first indication is that they swim funny or can't quite make to the top of the tank to get a breath of fresh air. They need to be able to do this, Ideally they would also have some plants in their display creating O2 for the betta's water while absorbing the excess CO2.
To fit in all this stuff you may need to purchase a larger tank than what you have now. Mine has 6 gallons to swim in which was a little overkill but I think they need 3-5 gallons at a minimum to fit in all the equipment without it getting in his way. There don't seem to be any other obvious diseases so I would start with improving his environment and getting him easy access to the air. You didn't say if he is eating? If he's not eating that unfortunately can be a lot of issues - swim bladder, constiplation, general disease ect.
He was laying on his side and swimming wildly around the tank once in a while. I put him in a little glass bowl....I warmed up the water in his bowl because it was way too cold (65 F.) and got it up to 80 F. but it's dropping again. His stomach looks a little bloated I think.... Here is a picture. Thanks for any help!
