Don't switch his food. Hopefully he is on kitten food. Watch his poo and pee. For signs of blood or limping. But if she took him to the vet hopefully all is well.
Just leave him be for a day or so. Maybe tomorrow intice him with a game of dragging a toy on a string or rolling a small ball around. In a day or so, he will figure out that pain is not the norm for his new home and come back around... just don't force it.
A good idea is to get him a bed of his own maybe one that attaches to the window or a cat tree that includes a place for him to sleep in safety.
I'm not sure if you are a new pet owner but here is some information on cat/kitten. We have really nice, helpful cat lovers on here that will always be happy to give advice. Care.
ASPCA: Pet Care: Cat Care