It's quite possibly his masturbation techniques. (I'd recommend a sex therapist over a regular doctor, by the way.) A lot of guys are really tight and particular about their masturbation technique, and it leaves them to not be nearly as sensitive when other techniques are applied. The best way to fix it would be to have your SO start to slowly masturbate less forcefully - by using a looser grip and everything. We can all accustome our bodies to a certain sensation, and we raise our "sensation point" to where things under that sensation don't feel nearly as good.
It also could have something to do with age and his penis not filling with as much blood, but if he's never been all that sensitive, that doesn't seem to fit.
Basically, I'd try scaling back the strength of his grip during masturbation and seeing if that helps. He may not orgasm the first couple attempts, but it's more to get the body responding to lighter touches.
I'm also not a licensed sex therapist or anything though, but those can get expensive and fast. x.x Most regular doctors will probably just prescribe medication though.