My initial guess would be to check Transair pilot shop or Sporty's, if they don't have it, I'm not sure it exists (and I'm sure you checked those). A quick Google showed some seatbelt extenders (including here;
https://www.aircraftspruce.com/menus...lts_belts.html) but no infant seatbelts. An extender is not what you want of course as it would crush your son in case of a rapid deceleration.
How about an infant carrier that your wife could use while seated, so that your son is seated on her lap but 'attached' to her, while your wife is strapped in.