this just happened to me for the first time ever. i can't invite him over, not even with my sim that was married to him lol.
One way to get ghosts to come to you is to give your home lot the spooky lot challenge (used to be a lot trait until CotL). A ghost will show up every night. From what I can tell they sort of rotate as to which ghost shows up and the one you are looking for might show up one night. You just have to be patient. Then you can talk to them, ask them to join the household, resurrect them, etc. But it'll only work if the ghost is still in the game and hasn't been culled. In your case, if the ghost still shows in the exspouse's relationship panel, then he's still in the game. If not, then he's probably been culled.
Hope this helps.
