Honestly I would be worried about hip dysplasia. Lazy puppies are often puppies that don't move much because it hurts.
I would also be worried about having two same sex, same age males together later. How much time do they spend apart?
My first Rottie Bruno was a large, lazy pup...he was diagnosed with severe HD by the time he was 9 months of age.
Another thought would be heart problems. I would have him thoroughly checked out by the vet.
Gina