I would just see how he does on a regular quality adult formula with no extra feedings or excessive amounts. He may have just been fed poorly in the past and that's why he's thin. If he fills out and seems healthy with just that then you know it was just poor care he'd received in the past.

Autumn does terrible on lamb but fine on chicken. Each dog is individual. I wouldn't buy a huge bag of any food you decided to try for the first time around.