My puppy was 50 lbs at six months and is now 65lbs at nine months- they continue to gain weight into their second year. If he seems underweight you could try to increase his food a bit, but if his weight seems ok, his growth is probably fine. I think the sluggishness is more concerning. If you think his energy level is abnormally low and he is not just a mellow dog you may want to bring this up with your vet. Some puppies are very mellow, my cousin has a golden who was an extremely mellow puppy. She took him to the vet when he was a few months old because she was concerned about his energy level, her vet did some bloodwork and everything checked out fine. He is still a very mellow low energy dog.