Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am so sorry to hear about Sebastian's sudden right, rear leg lameness. There are a couple common conditions that we can see with big dogs that cause lameness in his rear leg.
With a history of sudden lameness and no swelling or pain when touching the leg I would be highly suspicious of a cruciate ligament injury, either a partial tear or a rupture. The cruciate ligament crosses the knee it keeps that joint stable. If it tears there is no stability to the joint and the secondary inflammation is quite painful. it can happen suddenly with the smallest slip, especially one that causes a twisting motion to the knee. Chows tend to have a very straight legged rear leg conformation, so they are predisposed to this injury.
Ideally surgery would be done to give his knee normal stability because without surgery secondary arthritis formation will occur sooner and to a more severe degree then if surgery is done. It is also more likely that he will rupture the cruciate ligament in his other knee because he will be putting more stress and strain on the other leg.
With very strict rest the knee will form scar tissue and gain some stability with time but it won't ever be normal and it will be arthritic. When I say strict rest I mean cage rest, no running, jumping, climbing stairs or playing for at least 6 to 8 weeks. He needs to go outside to eliminate on a leash so he is not overly active.
Another possibility given his breed is that he is suffering from hip dysplasia, which is a malformation of the hip joint and secondary arthritic changes. Usually though these dogs are quite painful. Diagnosis is via radiographs under anesthesia. Treatment can be surgical or medical therapy to control arthritis formation and pain.
Ideally he should see his veterinarian for an examination and possibly some radiographs depending upon what his examination points to. It sounds like you are planning on doing this already.
For either condition long term I recommend keeping him on the thin side, or weight loss if he is overweight to decrease stress on his knees or hips.
Long term for joint pain and to keep arthritis formation as little as possible whether surgery is done or not, I do recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). These work synergistically and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them alone to control pain and inflammation. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.htm
To control his discomfort now his veterinarian can prescribe drugs that are more potent. Veterinary drugs we can add include a nonsteroidal like Metacam, Deramaxx, Previcox or Rimadyl. If those aren't enough we can add another drug in the opiod family called Tramadol and/or another drug called Gabapentin.
If you want to read more about cruciate ligament injuries here is a link to an accurate article about cruciate ligaments: http://www.petmd.com/dog/conditions/musculoskeletal/c_dg_cranial_cruciate_ligament#.Ulop6M7n_IU
If you can identify which joint seems painful tonight then you can alternate warm and cold compresses for 10 minutes at a time on the affected joint. Cold will reduce inflammation and warm will reduce painful muscle spasms.
You need to keep him very quiet tonight. If possible no stairs, running and jumping. The less he does the better.
If you can wait to give him anything orally for pain tonight that is best because the prescription medication your veterinarian has for pain will be much safer and work better than any over the counter medications that we take. In fact acetaminophen and ibuprofen aren't used in dogs because their effective doses are very close to a toxic dose in dogs.
The only over the counter anti-inflammatory that can be used in dogs is buffered, enteric coated aspirin (like ascriptin). Aspirin does cause stomach and intestinal irritation and ulceration as well as clotting problems so should not be given for more than 2 to 3 days consecutively and should always be given with a meal. If you choose to use it watch for lack of appetite, vomiting, blood in the stools or dark tarry stools and stop immediately if you see those. Do not use aspirin if your dog has liver or kidney disease or a history of a sensitive stomach or clotting problems.
The dose for aspirin is 5mg to 10mg per pound of body weight orally every 12 hours (about one 325mg aspirin for a 35 to 70 pound dog every 12 hours). Always give with a meal. Do not use for more than 2 or 3 days.
Be aware if you choose to use aspirin and it doesn't help your veterinarian will be limited on what they can give as ideally there should be a 5 to 7 day washout period between different nonsteroidals or nonsteroidals and steroids.
Please let me know if you have any further questions.