If he is used to eat vegetables, some wet grass would be good to encourage him to eat more and hydrate the guts. Some wet parsley, mint or basil might help to. Like the others said, the important thing is that he eats lots of long fibers and keeps his guts really hydrated. If he were mine, I would buy several types of hay, give wet grass and syringe feed a few ml of water several times a day. Do the poos look normal or are they tiny or misshapen?
If they look normal, it means it should be fine and that the guts work OK. If not, the guts have slowed and you will need to keep a close eye on him and make sure the fibers keep on coming to push the whole mess outside (rabbits' stomach don't have muscles like ours do, so they need strong long fibers to push things toward the exit, that's why when they don't eat enough hay blockages occur).