I had a litter(one and only) and the smallest boy was 2oz at birth,he was slower to open his eyes,slower for the teeth to come in and had to fight to get his turn at feedings.BUT! boy when he did grow he outgrew the others in every way.Coat,size,personality and he even taunted the others that he could get on my kitchen table looking for scraps and they couldn't
he matured to 5lbs at 14 mths so you never know. If he is healthy and eating even if its not the same amount as the others(he is the smallest right now and will require less food) he should be fine.
ALSO what did the vet say??