Previcox is actually a very good medication for arthritis and other causes of pain and discomfort in dogs. It is much safer on the GI system and joints themselves than aspirin and there aren't other safe OTC human products that I would recommend for dogs. Just Answer notwithstanding, there is a lot of misinformation on the internet! : )
That being said, there are some other options to think about. First, arthritis generally comes on gradually. If Scamp is suddenly showing pain, I would be concerned about something else going on--Lyme disease, ruptured cruciate ligament or meniscal tear in his knee, spinal issue, bone cancer, low thyroid, etc.
If Scamp does have arthritis, then you could ask your vet about trying Duralactin for awhile. This is a nutraceutical made of "hyper-immune milk protein" that helps with inflammation. I've had some good success in dogs with arthritis, at least initially, and there are no side effects.
There are also prescription diets specially formulated for dogs with arthritis. I feed my own dog Prescription Diet J/D and have a few client dogs on it and also Iams Joint Diet. They are only available through your vet. They are extremely high in fatty acids (more than you could supplement with pills easily) and also contain some glucosamine.
That leads to trying Dasuquin or another glucosamine/chondroitin sulfate medication. I like Dasuquin because I know it will work (in those dogs which respond to this nutraceutical). It does take 6-8 weeks to see a response, but certainly something to start now. If Scamp seems better after 6 weeks on it, when you refill you could try a human OTC glucosamine supplement. They don't all contain what the label claims, so it is better to start with something that we know works in at least some dogs.
Acupuncture and chiropractics might also be beneficial to Scamp. There are veterinary acupuncturists around--your normal vet may have a recommendation.
Hope that helps!