I would get a good quality wet. Merrick Dog Food makes an AWESOME wet food, it has whole veggies and meat. Wingling is nice, it has a whole chicken wing (but you must mash up the bone). They also make Grandma's Pot Pie in a dry, I'd get both and mix some together....I bet he would be in HEAVEN!
Honestly wet is good b/c it has more moisture and is less processed than dry.
Good for you! So glad you are helping this guy. With his past unknown and being so skinny, I would really focus on getting him to eat rather than holding out on him...Serious try my suggestions of the Merrick food.
Merrick Pet Foods : merrickpetcare.com