As someone with rsd in her inner ankle/foot/calf there is no way I could tolerate this even on the knee. I bump my knee and it effects the whole leg and sends the pain so high. You may want to get others response on the rsd forum too. Has he tried a warm bath? Or maybe even epsom salt bath?
Melody hope you are finding it helpful and your husband too. I can see you are being a great care taker like usual.
I think Walmart for many things like otc meds and vitamins is much cheaper then cvs or a drug store