Taking them to the vet is not a bad idea, BUT what you are experiencing is not that uncommon. It is still a good idea to rule out anything medical (tapeworm, heartworm, etc.).
But in the meantime, here's what I would recommend:
1) Bump up to at least Purina One if not proplan. You can still get purina one at the grocery store, so it's at least convenient.
2) Break up feedings into twice a day if you aren't already... For thin dogs (we have one right now, same age as your's. Male that is just growing like a weed), we'll do 2.5 cups in the am and 4 cups in the pm. That seems to work for us. It seems that if you do it all in one sitting, the digestive system doesn't work as efficiently... Just my observation, NOT scientific.
3) If you're looking for a bargain weightgaining supplement, go with corn oil... You can add 2-3 tablespoons with their food. Many years ago -- and it's been a while, although I used to share this all the time so my memory isn't quite as good -- Auburn did a study of various oils and found that corn oil was the one that was best retained as fat.
4) I would wait to switch to adult food until you have him at a good weight.
And finally:
1) check his stools. If they are firm and don't have any unidentified objects in them, that's a good sign.
2) check his coat - is it shiny and healthy looking?
