[Hydro1]

I highly doubt its hd, especially if the parents were tested and cleared.
His age is irrelevant. I run dogs that are 7, 2, and 5 years old day after day for cat season.
Is the stiffness confined to the leg that was previously injured?
You said legs, both front legs seem stiff?
Stiff as in walking like he has casts on his legs? Keeping them straight?

How quickly have you "amped" up his exersice?
Even changing a dogs routine from 2 to 3 hours of hard exercise a day can cause problems.

As has been said, must be nice to have a vet in the family.