Rocky was 45 pounds at 5 months. At age 3 he is 74 pounds. Ollie is on the small side. I would talk to your vet, and maybe check with the breeder and try to find out what size others in his litter are. First two pictures are Rocky at 5 months and final photo at age 3 years.
Yes it is a bit on the small side but my Ollie was also small like that as he was the runt and now at 2.25 years old only tops out at 53 lbs, I asked the vet numerous times about it and since she knows the breeder was not concerned as his dad was a smaller, Mom about average and none of his siblings were over 65 lbs who continued at this vet (5 out of 11) I like his size as my other conforms more to standards and is 75ish lbs. Ollie is the one on the right, Stan the left.