This could be a whole host of illnesses caused by a whole host of problems. Here’s just a couple I can think of off the top of my head:
-Water quality problems potentially caused by a lack if cycling (what are your water parameters?)
-Bloat
-Tumor(s)
-Drastic change in water quality/temp
-A contagious illness from the breeder/pet store
Honestly, though, it sounds like he’s in an state beyond saving. A common method of ethical at home euthanasia for fish is to remove them from the tank to a small separate container and slowly add in a heavily mixed solution of clove oil to the water. If you need more details on this let me know; if you do it incorrectly you could risk killing your fish in a less-than-ethical manner.
We got his water tested today, and they said it looked perfectly fine.
I also have some other things I forgot to include: it looks like he is breathing heavily, we put a chopstick thing in the tank next to him to see if he would move and he swam up to the top of the tank, but than he sunk back down again. Also, now the middle part of his body is turning white.