Stiffnes in dogs after training or working?

[wildside2014]

I have a very healthy 2.5 year old Chesapeake Bay Retriever. The past couple weeks ive been really amping up my dogs preseason workouts and fitness drills. Pup has been having bouts of real stiffness soas i can tell, in the evenings following a heavy workout. Im a firm beleiver in training a dog in higher intensities during the summer months if youre going to expect them last entire days during the fall season. Maybe im going too hard on a 2.5 yr old dog though? When i say amping up, i mean instead of just retrieving drills ive added in simple fun frisbee games with him, just to keep things fun and get him doing some hard
Sprints. I feel bad for the pup watching him get the stiffness. Should i scale things back and be even more gradual working him into agility stuff? I was thinking of adding additional glicosamine to his diet. He is fed well, and trust me when i say he is well taken care of. He is no stranger to being in shape. I myself
Run a lot so its not like i suddenly got him up and out. Should i be worried about hip dysplasia at his age knowing the parents were free and cleared?

Any suggestions from the active dog owners?