Have you tried
testingcheatsenabled true
Sliding the relationship meter to max
After that seeing if you can pick up his grave "If outdoors" or urn if "Indoors"
You can pick up graves of high relation sims.If so after that place it again in your yard or house and see what happens.
Secondly if that does not work when you get a visual of the ghost out and about with testingcheatsenabled Hit ctrl+shift+c then type in
Resetsim Bob Jones
Whatever his name is in place of Bob Jones then hit enter
If he moves or dissapears altogether then try interacting next chance you get