Hmm..I know all about the picky eater! Scottie was also picky since day one. He used to pick out the first two kibbles and spat it out on the floor (always reminded me of people who go wine tasting and would spit out wine). I also hand-fed Scottie for the first year, for two reasons: I used his kibble to do daily obedience training (because he had a sensitive stomach and couldn't handle most treats) and second, he would stare at his food and often walk away so I had to make it 'fun' when it was his dinner time.
Anyway, if Frankie was eating kibble regularly, I would just stick to that. Forget the chicken. Don't break what's already working. If he's eating the recommended amount per your dog food bag, he should be fine. They usually continue to bulk up, even after they reach their full size. I recall Scottie finally feeling a little rounder after 1.5 years old. He was always quite lean.
I also recommend feeding him at a regular schedule. That usually helps as dogs are such creatures of habit.
