Also, if he's having pain he might want to see a doctor. And in any event, unless we know what is causing the pain we can't give advice. Is it just fatigue from bottoming out? Then o-rings or stiffer springs might work, or topres, or alps, or... Is it something else? Then who knows.
Anyway, from what I understand about the Matias quiet switch, it has about 70g actuation force and is designed to bottom out. The impact is probably cushioned slightly by the dampeners in the slider, but not by much. I have the Apple Extended Keyboard II which the Matias is "inspired by," but I don't know how close the actual switches are. But I really like the feel of this keyboard, and t causes me no pain or fatigue. Or my brain fatigue grows faster than my hand fatigue, so the keyboard isn't an issue. At any rate, I hear great things about the Matias Quiet Pro, but I don't know if it's right for someone with a vague medical issue.
WASD carries special soft o-rings, which might be worth a try. Putting them on some reds might work.