Skinny German Shepherd?

My Shepherd just turned a year. He was the runt and has always been kind of small, even though everyone who sees him says "wow he is going to be a big boy". He is long and has always been on the slender side, but still looked healthy. He has had food allergies since he was a pup and it took me a long time to find a food he would not throw up. I recently changed from puppy to adult food and found something that seems OK, but over the last couple of months he looks too skinny. You can even see his ribs a little. He eats but not a lot. He is still very active and energetic, but it worries me. I tried adding some wet food to the dry and he threw up really badly, even acid, after he lost everything he ate that day. I am back to straight dry food, lamb and rice seems to sit well with him, but is there something else I can do or anything I should be concerned about? Please help. He is my baby and I don't want anything to happen to him. He is up to date on shots.