arthritis can be helped, certainly the anti-inflammatories are the best thing for the pain and inflammation, also glucosamine powder, fish oil, omega 3, shark cartelidge or rather then purchasing all of these supplements separately Hills prescription diet J/d for dogs with arthritis contains all of these ingrediants.
There are arthrtitis injections that the vet can administer to your dog called cartrophen, these injections prevent further cartelidge breakdown and also help to repair the cartelidge. these can be extremely beneficial.
try to avoid excessive exercise unless it is swimming, if you can get your dog to swim this will be very beneficial

You should probably look into hydrotherapy:
http://en.wikipedia.org/wiki/Canine_hydrotherapy

Make sure he’s on a high quality kibble that is fulfilling his nutritional needs. It should be designed for large breed adults and should include glucosamine.

You should ask your vet about the possibility of surgical reparation, which is a common course of action when the dog is suffering damage of the cranial cruciate ligament. I don’t know why surgery wasn’t brought up as an option, so you should take the initiative and ask about it. You should also ask about an anti-inflammatory pain reliever, such as an NSAID like Rimadyl. (These can have negative side effects, however, so don’t jump right into them without first investigating other options.)