Janelle, phesina is right - just tuna by itself is not enough nutrition for a cat.

Get a good quality food - just dry food is ok - and get some hairball preventive from your vet. It comes in a tube, and is malt flavoured. My fluffy guy gets it mixed into his wet food, but some cats will lick it right off the tube!

If he doesn't like Science Diet, there are other brands - MediCal, Royal Canin - that some cats like better.

Don't give up because Simon didn't like the one brand.

Ditto on the second opinion.