Found a cat today outside my house, very skinny...?

It it a very fragile cat, pure white, he seems too not cat like a normal cat at all, very sweet and nice... but he barley eats anything, and hes really bony... he eats a little food here and there, and when he goes drink water he sits there for 20 minutes then goes lays down. He seems amount 3-6 months old. I was wondering if there are any ways of encouraging him to eat? I bought him some kitty soft food, he barley ate any... I also gave him some hard cat food, he ate a little.. but I really think he needs to fatten up. Suggestions?