If you work that out over the 5 weeks though, it's just under 300gms per week - which is a great gain and is normal for a baby his age. Don't forget too, that at 3wks he would have gone through a growth spurt and gained a little more than usual too. He will have another growth spurt shortly too, so expect that the next time you get him weighed that he will possibly have had another significant gain. My youngest was like that - he stacked it on for the first 8wks and then he tapered off and had smaller gains each week. He (and you) are doing great!