I have a cat and I usually feed him tuna and dry cat food. I wanted to know what other foods I could, or should, feed him for him to be healthy.
Cats are carnivores, they have to eat meat in order to be healthy. In regards to your current feeding:
Tuna in the can for human consumption is generally the muscle of the tuna, not the organs. Cats need to have both to get the nutrients they need. So, strictly speaking, tuna alone is not enough.
Dry cat foods are often loaded with vegetable based fillers. It is important to really read the ingredients on the food to ensure that you're getting dry foods that are mostly based on animal proteins.
The biggest risk for cats with insufficient animal protein in their diet is the lack of taurine. This can cause some pretty severe issues in their health, including blindness, hair loss, and tooth decay. So, how can you make sure?
Very high quality dry cat food should have the required nutrients. Just double check. If this is the case, and it very likely is, then you probably want to be sure that the tuna eating isn't excluding a good daily intake of their regular food. Keep it as a treat.
Look to supplement their diet with wet cat foods (I have reason to believe that you already do) as they are less likely to have vegetable filler. However, it needs to be a good food, check the ingredients.
When it comes to cat foods, as a note, please really pay attention to the ash content and the nature of it. I learned, the hard way, that too much can be very fatal, very fast, especially to male cats. It really is within 24 hours, I can sadly attest to that.
For some good, if a bit loaded with technical vet jargon, see: Cats: obligate carnivore. You may need a translator. :)
At any rate, assuming you're in good standing with all of the above, then there isn't other foods for you to add. Cats don't like variation (with reason), so if your fellow is healthy and happy, you have no need to change.