We've covered quite a bit of ground; Sam Miller III's post got me mulling over this thread from a different perspective. I'm early 50's, chunky with a larger cross sectional area than ideal, and I don't do well swimming into current, so I can relate to your husband potentially benefiting from a gadget that can give him a boost once in awhile. If it's small enough to go in checked luggage and clip out of the way on a D-ring when not in use.
In other words, I took it to be a convenience, not a necessity.
You mention his kick's less strong than it used to be, and the lack of such aid will close off a lot of potential dive sites. That sounds a little more significant. Maybe even potentially a necessity.
I'm not judging your husband's fitness to dive; that's his call. I am curious as to what areas, or types of areas, you guys dive, and what places you want to go that seem out of reach without such aid. Cozumel? The occasional Key Largo dive where you might get some current? Deciding to call any boat dive where they figure on some current and decide to do it as a drift dive?
If we're making suggestions about what might help him make dives that push his physical limitations, I'd like to know the destination and types of sites we're talking about.