Chloe was around 40-42 lbs at 7 months; her sister was 55.... Chloe is tall and very skinny - too skinny - and we have had problems with her not wanting to eat. Out of 6 pups in the litter she was in the middle for her weight -- and all the pups now growing up hold pretty much true to their birth order (i.e. the biggest sister at birth is still the biggest

)
Do his hip bones show, and his spine? You said he eats & is very active - so that just might be it; just slower to put on the pounds :D. Do you know the weight/size of his dad? - that's usually a good indicator of where he will end up.
If he is eating, and is active, and happy - then I wouldn't worry. I noticed that they don't grow steadily all the time - CHloe slowed down at around 6 mos. and just now we're noticing that she is (finally) starting to put on some weight & fill out (a tiny bit). I guess they have their own grow spurts - on their own time :D.