I've heard of some dogs having a hard time gaining weight on EVO and Orijen and other grain free foods. Because of the high meat content, it kind of works like the Atkin's Diet...high protein, low carbs...and you lose weight.
Try feeding Innova...some dogs gain weight eating a kibble with grain. Innova is high in calories (I think about 556 calories per cup?)....or keep him on the EVO and give him some oatmeal or pasta extra. I used to fatten up foster dogs with an extra meal of oatmeal.
You could also add in a raw meal... a couple chicken legs with skin on would probably give a few extra pounds?
It just may take time for him to fill in and mature...some dogs just need the time...not the weight.
Gina