Strict doritos chip diet for a cat?????????????

i found my my cat about six minths ago as a stray. He was very skinny and i started him on a regular diet of dry food but he was not gaining much weight so i started him on a dorito chip diet three months ago. He has become lethargic sleepy and not his self. So about a week ago i started puting sugar in his water to get him some energy. What am i doing wrong!!!!!!!!