Did you get my email I meant to include a supplement regime for you to get him on. Glucosamine/chondroitin suppl. for dogs by Trader Joe's.
The bottle says for dogs over 50 lbs to feed 3 times a day (but I feed them together) and that comes to 750 mg glucosamine, 600 mg Condroitin and then it has 45 mg Boswella Serrata. Since I only give it as a preventer, I am not giving a high dose. I also use the Costco stuff and it seems to work. Yet, I personally am not to sure about anything with vitamin C at this point (unless it's in his food).
Chondroitin is what's most important to prevent arthritis.
Here are a couple of links to check out as well.
http://www.mastiffonline.com/modules...highlight=tplo
http://www.mastiffonline.com/modules...=hip+dysplasia