We have two cats how can we retrain our older male cat to eat dry cat food?

Our older male cat is about ten years old. He loves tuna. Both he and our female cat (she is about seven) love tuna. But they are very very spoiled and picky. Sometimes, the male cat will not eat his dry Meow Mix food at all. The female usually does, however. So how can we encourage the male cat to eat the dry food again? He only likes to eat the tuna! Also, I have heard that Tuna is bad for the kitty cats. Is this true?