From your description, it seems that the potential threat is likely to come to the home. Mace is possibly the worst home-defense weapon I can imagine. An untrained person who is startled awake, in the dark, is very unlikely to be able to deploy it effectively (especially if he is a limp-wristed, effete, ivory tower-type of individual). Even if he does deploy it at the intruder, in the close-in spaces of a home, there is a good chance he will expose himself, and be in the position of having to wrestle with a pissed off bad guy on the floor.
Of course, all of that assumes the bad guy doesn't have a more effective weapon of his own, like a GUN!
In my opinion, you would be better off bringing your son-in-law one of these: