You'll need to find a Vet that's able to work with exotic animals. Google vets in your area, read up, make some calls, leave some messages. If they can even work on a little tree frog he might just tell you that even if they're able to mend it and get it back inside his muscles there might pop it back out again sooner or later. For a four year old+ tree frog it might be best honestly just to put it to sleep so it doesn't happen again.
I wouldn't feed him at all until it either pulls back inside or you consult a vet. I had a toad once growing up that had a prolaspe and it died in the matter of a week, probally due to infection or blood poisoning due the inability to get rid of fecal matter.
You have my prayers in any case. Prolaspe is a terrible thing to happen to any creature.