Congratulations on getting your son relief! That's a wonderful win for both of you. A few things come to mind besides the fact that you should cross-post this to the
Magnus8 topicals thread for those who look there for topical oil info!
My first thought is that RA pain (or even OA pain), when not near the surface (knuckles, or my wife's OA big toe for example), are difficult to attack with a topical. Topical oils block the transmission of pain only in the nerve endings it can reach through the skin, and therefore something like hip or shoulder pain, buried under layers of muscle, will see little or no pain relief. The THC does not get into the bloodstream when rubbed into the skin (a good thing for those who get tested!). That's where the pain relief of
internal THC (and CBD for the inflammation) comes into play: smoked, ingested, or used as a suppository. [I'm not familiar with the interaction of RA and THC from a healing standpoint but others here may have more info on that.]
Where we do see great relief, as your son has wonderfully discovered, is topicals applied to the base of the neck/top of the spine. That's where a large collection of unmyelinated nerve endings reside near the skin and close to the spinal cord, allowing the THC to block pain more effectively than when applied elsewhere to the skin. You might even want to try it in that area to see if it can block the pain signals heading to the brain from your RA.
It's always great to hear the positive feedback from people who are getting relief with topical oils. As SweetSue said, topical oil could easily be what gets the rest of the world to accept the positive powers of this plant!
[edit: Also, stronger is definitely better with the topicals. I feel that a minimum of 10mg/ml is necessary. I aim to get closer to 15mg/ml.]