Is he a housed sim? I ask because I had a sim marry a former mail carrier in one of my saves and he lost that job role as soon as I placed him in a house. All the game-generated townies who've had this job in my game have always been in the "currently not in world" bin, so housing them seems to work to relieve them of that job and force the game to generate a new sim to take the role. If that doesn't work, maybe you could just move him into your sim's house, get him a different job (since I'm pretty sure making him a played character will strip him of the mail carrier job), and then have that sim do the proposing.
Edit: I just realized you were able to get them engaged. Never mind, then. :lol: Glad you were able to get it worked out for your sims! :)
