I wonder what medication he’s on. I used to take two short acting Dexamphetamine tablets and I would often forget the second one, now I’m on the long acting version (just one tablet a day) and I remember to take it on most days. I also used to get really, really cross when the fast acting medication ran out at the end of the day, but no mood problems at all with the longer acting one (the rebound problem of anger was why I switched drugs).
Maybe your fiancé can leave his tablets next to something he uses every morning like the kettle or his wallet. Then he’s more likely to see it before he leaves the house. Years ago a friend of mine used to blue tack her contraceptive pill next to the light switch in her bedroom, so she’d remember to take one ever night before going to sleep 