Try not to worry! It is not easy to have a dog with arthritis... but there are some ideas to really help!
I would in no way wait until the dog is in such pain that he cannot walk before giving him relief.
By then, it can be too late, as pain can overwhelm. Here are some ideas to help:
Right now, I would:
1. Find another vet less terrified by the possible side effects of drugs that will give QUALITY of life to an animal who is in pain already.
2. Begin researching what supplements you would like to give. Glucosamine and chrondroitin are starters! After 2 weeks of Cosequin (there are less costly versions, too), my senior was moving around much, much more comfortably. You CAN actually build a joint up, by giving it cushion and good fluid, using these supplements.
3. Find a vet who will rutinely check liver and kidney values on the Deramaxx, Metacam, or Rimadyl. They all MAY have side effects. It does NOT mean that there will be any notable harm to your dog. With blood levels and CBC monitored, this is normal for these meds.
4. You are a great owner for having brought your dog to the vet to begin with-- now you just need a vet who will support you in keeping your dog pain-free. Find a vet who listens to your worries.
My senior was on Rimadyl for 4 years, lived until 15.5, and felt GREAT after he began Rimadyl. It was better than him living in pain-- and he felt wonderful.
Hang in there-- you will do GREAT caring for your dog!