Bulges can come from internal parasites, especially if he is skinny and eating a lot. I would look into a good anti parasite food and then use something like melafix to help the sore heal faster
If you want to treat for internal parasites and he is still eating I would recomend this...I used it for my DPs when I had them for most it was to late because they had already stopped eating but it did help 2 of the 6 I had.
You could use it as a preventitive also it will not harm your fish, and may be a good idea for you. I am not big on treating without knowing personally..but this is somthing that could help so you may want to try. You soak his food in it before feeding that way you know it does get in his system.
As far as melafix you will get mixed reviews on it...some people say to stay away from it as it is just tea tree oil basically, but I have used it with success with fish in the past and would use it again if needed. The thing with it though is, it can be dangerous for scaleless fish (like plecos or most sharks) and fish like bettas that are Labyrinth. If you use it be sure to follow the directions exactly. If you chose not to use it though good water changes will really do the same for it.