I got to Sinjin, killed him, and saved Kent. Talked with Kent, and he tells me that he is going back to Goodneigbor. I keep checking, but weeks later, he is still not there.

Any help?

he may still be were you left him...he will give you a gift first before he goes back

He is stuck where you left him probably. Try constantly running towards him (it works the best while sneaking) so that he slowly reaches the elevator. After that he's going back.