I didn't even know that could happen to chinchillas. Well, not that I thought it couldn't, it just never crossed my mind if it happens or not. I've only dealt with sheep and goat prolapses (anal and vaginal). They always just get a harness (like a wierd skeleton-of-a-diaper like thing) that holds in whatever is trying to get out after they're cleaned up from the initial prolapse and get all the other routine stuff like meds etc. Obviously, I don't think they have that for chins and they would probabaly chew it off anyways hah.
Sounds like if you can just get his weight up a bit he'll definitely be on the mends. I'd say to hang in there and keep going through this last stretch but I have a feeling you will regardless of what I write. You've come this far (a month!!) and done your best, really the best anybody could have. I'd consider that a huge accomplishment by its self.