It sounds like what my dog had ,it's called cherry eye and the only recourse is surgery. It's a fairly simple procedure. the vet just stitches the lower eye lid back in place.
My Basset Hound's Eye Roles Back And Looks Like His 3rd Eyelid Swollen. Can You Help?
Dogs don't have 3 eyelids. You need to get him to the vet, there's something going on. Good luck.