
I have also seen this when the owners have been almost too careful and the pup has not got much excersise.
Muscles support the growing skeleton and also help in avoiding accidents as the pup is more "compact and held together".
I would say gentle off lead (or on lead first, if the pup is too exitable and might do damage leaping around) exercise in different grounds.
Some people use walking on gravel as it is supposed to make the pup pick up its legs and kind of stand on its toes more, strenghtening the leg...
Any forests near by? little hills (more like big bumps :) ) so he is walking up and down, using the muscles differently should help if they are not too bad.
The pool idea might be better is he needs to strenghten them up gently before doing off lead excersise (to avoid accident/damage)
Also, some giant puppy foods actually have less protein than the adult version so I would check this before even thinking of changing...
Edited to add, too much weight on a pup can also worsen these.
Have the owners been following how much the pup is gaining a week?
Are they aware that it is important to follow the pups weight and that he is not gaining too much per week as his skeleton might not be able to keep up.
I know experienced owners will be able to see is the pup looks ok but for somebody not so experienced skales will help to see the puppy is growing nicely and evenly.
So rather little light puppy than too heavy as this will ease the pressure on this "bend" legs.