Questions about glucosamine

I would be greatful for any info that anyone has on dosing. A quick history: We adopted Paco after he had been hit by a car and broke his right leg(ulnar bone fractured once and radius fractured in 2 places). He had some difficulty healing because he's a naughty boxer and kept jumping on his cast and wouldn't stay off it in his foster home. He's fine now but both the shelter's vet and our vet say he will probably develop arthritis in his carpus and elbow as he ages. He's about a year and a half old now. The only time I see it bothering him is when we take him to the dog park (when no other dogs are there to play) and he runs off lead like a horse at full speed for a long time. He tends to favor it towards the end of his run session but doesn't limp or favor it later while walking arround. I would like to get him on some supplements to try to prevent this and any future problems as much as possible. His food has listed on it's guarenteed analysis 300mg/kg glucosamine and 100mg/kg chondroitin. Is this adequate or should we be using supplements also? If so how much? He is about 70 pounds.  I realize this is long so thanks for reading and any info that anyone has would be really appriciated.