What is a normal weight gain per week for a Saint pup?

At what point does the weight gain begin to slow down? Just curious where we'll top out with this guy. My only concern is his health, weather he tops out at 100lbs (unlikely, I think) or 200+lbs. Buster is my first experience with a giant breed, I grew up with dogs in the 50-90lb range.

Buster came home at 9 1/2 weeks, weighing 20.4lbs. We weighed him this afternoon and he's up to 47.2lbs at 15 1/2 weeks. This puts us averaging just under 4 1/2lbs per week.

He was under weight at our first vet visit due to a "massive parasite load". The vet was amazed how bad his case of intestinal worms was. 6 weeks later we're still fighting it, he was given a worming pill the first visit, Panacur 2 weeks ago and we started another round of Panacur today with one more round starting 2 weeks from when we end this round. He now has a little fat padding on his ribs, can still easily feel each one (wouldnt be able to see them even if he was emaciated due to his thick fur).

He eats 4-6 cups of food per day, spread out in 3 meals (not every meal is finished). Treats very each day, only thing I do daily is his large milkbone at bedtime (lure/reward for sitting while I put up his baby gate).

As much as Im enjoying his puppy days, I cant wait to see my boy fully mature.