Multivitamin?

My cat is currently on Science Diet sensitive stomach, ever since we got him he had diarrhea and we tried Wellness, Pro Plan, and Nutro and they all upset his stomach, but Science Diet has really been helping out, the vet and I think it might be the higher meat contents that upset his stomach, anyways, my question is should I give him a multivitamin? I want my cat to have the best I can give him, I'm not too excited that I have to give him Science Diet, but it's what he's been doing the best on, so I was wondering if he really needs a multivitamin since he's on a commercial food, or if he would benefit from it.
Thanks!
Mandi.

This is a vet question... SD IMHO is not great but it does pass AFFCO for being complete and balanced .... that means that it should be fine for most cats

Okay, thank you