He does seem underweight for his age, assuming pure bred & all. I mix adult & puppy food and have it available at all times. You may have a special case if trying to bulk up the pup some. The idea with food always available is they don't develop the need to scarf the food so quickly.
With very yummy food they will wolf it down. You could try some quality dry and he should slow down some. Even if you want to continue the 'special' food, you could have some less palatable food available. Our dog will inhale a training treat, but eats her regular food rather civilized.