My Dog Was A Stray And Took Up Residence With

my dog was a stray and took up residence with me. outside dog, was very skinny, I started feeding him, he was anti social, and after 3 months I could finally pet him. I've had him for two yrs and in the last 4 maths he has lost weight but has a large belly. ribs and back bone show but belly is big. I have wormed him and have used flea and tick control. he is looking worse every day. what could be his problem?