He's a cutie for sure. If the vet said he was healthy I wouldn't worry. They are all different. They grow at different rates. He will have a spurt and grow soon enough.
My girl was 4.8 lbs. at 7 weeks. She is now 60 lbs. at 11 months.
There was recently a thread comparing weight and age of everyone's dogs. Can't find the link (maybe someone else can help with that).
If you find it you will see how much our pups very in size at different ages.