There isn't an average weight for a puppy that could even be close to accurate.
If you are feeling his ribs he is probably down 1-2 pounds of his weight.
In the above site the pup pictured was 12 pounds at about the same age as your boy but was a female. That could mean your boy is not going to be a huge male, could mean your pup is undersized.
But there isn't an average weight all there is would be a weight for your dog at his growth stage.
For example I'm raising a giant breed puppy now. In her litter there were 6 puppies. Two of the males were double the size of their smallest female littermates. One of the females was double the size of the others. One male was barely bigger than the females. So 'size' is relative to the individual pup not to a norm.
Things I can't tell over the net include how much bone your puppy has, how big other pups in the litter were, how large the parents and grandparents were, how well fed the mother of the litter and the litter were etc.
What you can tell is that he is underweight for his frame now if you can feel every rib and his spine.
Adult weights in this breed go around 100 pounds or over. As you have no idea how big this pup will get or how his particular blood line grows making any guess at the 'correct' weight just won't work.
Normally in dogs you can guess adult weight by doubling the pup's weight at 16 weeks but in my experience the giant breeds go over that by some margin.
What you should see now the worms are taken care of is steady weight gain every week.
You want him fit and not fat based on how far his frame has grown.
This site on the breed
says that at 6 weeks their pups weigh between 12-16 pounds.
This does not mean you should try to make your pup grow faster, get fat, over feed it etc. Instead you need to feed your pup so steady slow growth goes on. Forcing him to grow fast will lead to more problems than letting him grow slow and steady. He will hit is genetic potential either way but with slow growth may be able to walk and run better for life.