was the sore there before he groomed hard on that spot?
Gliders will notoriously aggressively groom flaws on our skin. Freckles, scab, pimple, mole, tattoo, anything that seems out of place to them they try to remove they do the same with each other which is why we HAVE to separate them if there is an open wound.
They aren't trying to hurt each other or make it worse they are trying to fix it, and make it worse in the process.
Have you tried long sleeves to cover your arm where he grooms, or maybe a sweatband?
The back of the head prolly the same way, have your spouse look at your scalp for something you may not even know is there like a freckle or mole or something especially if it seems to be the same spot all the time