If worse comes to worst, Bmore, clinch your cat's jaws open with one hand, and then give him a big spoonful of cod liver oil with the other. You'll ptobably need to hold him tightly between your legs while doing so. I know it sounds cruel (although I've seen vets do it several times), but if he can't eat wet food (why not, by the way?) and doesn't naturally drink much water (which is a persistent problem with cats), I don't know what else you can do. (Perhaps if he won't eat tuna, you could entice him to eat some sardines or salmon drenched in oil. They're both naturally oily fish to begin with.)