Did you check the height and weight charts?  What did he weigh when you brought him home?  I think the rule of thumb is 3x weight at 8weeks and 2x at 12 weeks... 
I am sure he is healthy.. so unless he is way off the charts and is eatting I would not worry. 
Saying that.. I felt the same about remy - she is skinny!! She did gain a couple of ounces the last month (she is now 1 year old) and is almost ..but not quite... 4 lbs.  The vet said she is fine     
				__________________  Shinja  mom to   Remy   lil Sis to     Bailey and Sammy        |