What's wrong with my betta fish?

My betta fish has just been lying around for quite awhile. I estimate that he is about 2 years, 7 months old. I got him over a year ago. He has a filter and a light that keeps his tank in the range that betta fish require. I clean his tank every 2-3 weeks depending on how dirty his tank is. It doesn't really get dirty which is why I don't clean it every week like most recommend and I put a tap water conditioner in it. I got him from Walmart. He has a 1 gallon tank which I read in a betta fish book is a good size for a single male betta which is what I have. I used to feed him flakes in the morning and blood worms at night, but then he got kind of swollen a couple of times and I posted another question on here and someone answered to give him a pea which I did and he got better. That's happened a couple of times and I've gradually made it so that he only gets a small amount of flakes at night so that he doesn't get swollen from overfeeding again. But ever since that first incident of him becoming swollen, he has just been lying around at the bottom and at the top of the tank, sometimes on his side. He never really flares or builds bubble nests either. I've seen him flare once and he might have started a bubble nest once, but when I went closer to see what he was doing, he swam away, so I don't know. Does anyone know what's wrong with him? Please help! Thanks!