You can soak his behind in sugar water to shrink the prolapse.

Rectal prolapses generally occur for three reasons: too large of food/prey, constipation, and/or parasite infections.

A vet is a good idea. Not only can they test for parasites, but if they can get the prolapse to reduce they can add a dissolving stitch.

Do not feed for a few days and only feed soft foods like worm pieces or Reptaboost once the prolapse reduces. Large, harder foods can make the prolapse worse.