How much should I be feeding an underweight stray?

[The Stray Cat Novice]

About a few months ago, a little orange and white tabby had been showing up in my garden, meowing like crazy. Of course, during the fist few days, I had been ignoring him. Why? Well I was way too dumbstruck back when I had few knowledge of cats, questions had been flooding in my head like- what if this cat has rabies? what if it gives me a parasite? I'm not ready to die yet!! You know, basic questions selfish humans ask themselves. But oh boy, my scared mentality deteriorated as soon as that cutie began getting closer and closer to me. I know, I know, I shouldn't have refused him food in the first place, and I really regret that, especially when he let me pet him and I felt his ribs and backbone on my hand. It broke my heart. I quickly served up a can of tuna for him but as soon as he finished, he meowed for more. I knew I didn't have any cat food, but basic knowledge told me that cats were carnivores, I should be feeding him meat. But what common sense didn't tell me was that I shouldn't be feeding him manufactured meats with a bucket of salt. But at that moment, I thought, who cares? He's hungry and he deserves all the meat I have.. So yeah, I fed him a tuna can with an entire package of sausages.
Okay, I know that wasn't really responsible of me. I could've just fed him another can of tuna or cooked him a piece of chicken, but it finally clicked and I felt so terribly sorry for the starving cat who had been waiting at my doorstep meowing for food, probably feeling betrayed as I simply looked at him and moved on, so my instincts told me, give him the sausage packet, no, not just one sausage, the entire package.
Now he comes by, every morning after I drink my coffee, and every afternoon after I take my daily walk, meowing something like "ah yes, my servant, I shall meow loudly now, so you know to give me a can of that delicious tuna".
But the thing is, I'm worried that the little guy will get sick with too much salt in the tuna, and with quick research I found that tuna should simply be an occasional treat, not a full on breakfast and dinner for the kitty. And the other thing, I don't know how much to give him, but he hasn't been putting on weight after all the tuna I've given him, so I know for sure he's still underweight. So all you cat connoisseurs out there, could you suggest what foods, basic human foods or cheap cat foods to give him, as well as how much he should be eating? I'm pretty sure he's at least four-to-five, maybe even 6 years old since the guy is a lazeball and barely grooms himself after eating. Oh well, I'll see what I can do about him, maybe even keep him if my mom allows it! But again, feel free to answer, and thank you if you do!

 

[Kieka]

Cheap cat foods will be better than human food. Cats need the vitamins and minerals from the whole animal and most human foods aren't whole animal. You can usually get Friskies or 9 Lives canned foods pretty cheap. Just go with pate, no cuts or gravy lovers versions. They aren't top of the line cat food but it will have all the nutrition the cat needs and won't break your budget to get them. If you shop sales you can usually find them for a pretty good price that might even be less than the tuna can.

 

[The Stray Cat Novice]

Thank you so much for your reply! I was actually heading out to the grocery this weekend so I'll be on the lookout for the brands you mentioned!