By and large (no pun intended), substance and mass are genetic. You can't create it. You can enhance what is there.
You can roadwork your dog when he's old enough and make him the most fit athlete he can be but if that means he's in his best shape at 90lbs, 4 yrs old. So be it.
Resist the urge to overfeed, over supplement and resist the desire for him to "poof" into a 120lb male. He sounds fairly small for his age and may quite likely not be a big guy - ever.
