@dandydog I completely agree with @tanza. Don't worry too much about weight. Pay attention to the body. You don't want him too fat, but you also don't want to see ribs. If you can see ribs, he may be too thin.
My litter last year... we had one female who lagged behind the pack for the first six weeks. We paid close attention to ensue she was thriving and gave her first dibs at the tap room. In other words we would wake her early and get her to momma before anyone else to make sure she got her share. She always lagged, but she was gaining at a similar rate as the other pups. Today she's as big and healthy as all the rest.
It is a genetic lottery of sorts. Some are BIG like our first Basenji, Jengo, and some are smaller like our current Princess, Sparkle.
I'd be watching to see that he's gaining weight. If he is that's a good sign. And again, just make sure you can't see rib outlines when you look at him from the side. Run your fingers over his rib cage. With a light touch, if it feels like a roller coaster you might need to increase his calories. If not, you're good and he is too.
As always, if you're really concerned... a good vet is a never a bad idea. If you can, post a picture so we can see him. Side on would help.