IMO: buy a holster for it, and semi-permanantly mount it in a very unusual/unsuspicious place that is out of sight.
A Blackhawk Serpa holster is great for this use, because you can drill 2 holes in a cabinet or whatever piece of furniture, then screw the holster portion to it.
If he might consider quick access, then inside a drawer bolted to the TOP inside is a pretty decent spot.
If he has a dresser or something diagonally in a corner(so there is a "triangle" of open space behind it), and you can reach behind, then maybe screwed to the back side of the dresser.
You can always stick it in one of the more unlikely places one would look: the cabinet under the kitchen sink. There is generally a "blank" panel at the top because there is no space for a drawer(because the sink is in the way)... well because the Serpa holster has the retention, you can screw it in so it's pointed nearly straight up, open the cabinet door, reach upwards and release it.
He could always hollow out a book and stick it on a book shelf. Assuming there are other books for it to be disguised with. One book might look kinda strange.
If you are trying to keep kids or others in the house out, then a gunvault or similar is a safer bet. Still preferably hidden out of sight.