Why is my cat skinny?

Hi, I have a 2 year old cat. We adopted him as as stray when he was about two weeks old. He eats two bowls of kibble a day along with two pieces of raw chicken/a packet of wet food daily. He is picky and doesn't eat eggs, milk, or a uthi else actually. Occasionally he hunts birds and rats but I don't think he eats them (he threw up a pigeon last year and since then I see him hunting but not eating). He is very active, loving and even noisy at time when he wants attention, food or water. He usually stays out from 1-5am, sleeps through the day and potters around the house the rest of the time. But - he is very skinny.. Barely over a kilo. What could be wrong? How can I get him to gain weight?