See if you can find any meds that work well and treat multiple illnesses (particularly ones you think he has). Be careful with it, though, and ask people you think you can trust where you buy them if there are any. Just be wary of input focussing on solely more expensive products, because they might be trying to make more money off you, even if it isn't the best solution. If they reccommend multiple things and/or their go to meds aren't as expensive, then they may be more trustworthy.
And Leeniex is right, he is on the thin side. If he does eat okay, how much are you feeding him? You should feed him 2-4 pellets twice daily and skip a day one or twice a week for healthy regulation.