Supplement for cat w/ IBS?

[dogmelissa]

I have a kitty who has what I call Irritable Bowel Syndrome. He gets NASTY diarrhea if he eats anything other than what he's eating right now (Hills Science Diet Sensitive Stomach). He's also a bit overweight, so I've been mixing in some of the Hills Light, and have had pretty good success at keeping his weight under control as well as keeping his bm's in good shape.
However, there seems to be something lacking in this food, or perhaps that he's not absorbing well from this food, because his coat is in kinda gross shape. He's got flaky skin, sheds more than I think is necessary, and it's not as shiny as it used to be. He does like to be brushed, and I do that on a fairly regular basis, but that doesn't seem to help. So I want to see if I can supplement something into his diet that won't make his poops get gross, but will help his coat. Suggestions? Either a vitamin, a treat, a home-made something... I'll try anything at least once.
Thanks in advance!
Melissa