I think mine was around 20 lbs at that age, but I wouldn't worry about the weight. As long as he looks healthy and vet checked him out, he should be fine. Typically they gain about 2-3 lbs a week during this time. Different dogs, have different growth spurts, some early some late. Feed a high quality food. If he gets too skinny, feed more. If he gets too heavy, feed less. Look up the condition chart, he should be at 4 or 5 on a 10 point scale.