Just a suggestion mate, as there are many variables, but maybe try keeping him on easier ground and restricting the exercise for a while, not easy I know. See if they start improving.
Just to add, as I'm sure that some people will already know, before I get stick for this, but most running dogs especially the bigger types, take a fair amount of time before tendons, ligaments etc are fully set.
Anyway, hope he comes good for you ?