Ideas
Thank you for the information about him being drowned, I didn't remember that. I knew it was a bug, but he's been alive for so long that I thought it was now part of the lore and that someone had brought him back to life XD I didn't see this fix come through, but I'm delighted about it.
Well, wait. No, him being ALIVE is the bug. Unless maybe you don't have the Life & Death expansion maybe he's not alive anymore, but it seems that he IS still bugged, at least in my games. I added him to a household and... he's alive. I looked at his traits, and he does still have the "Dead" trait like he should, however due to the changes made with Life & Death, that's now basically an invalid trait. In fact, from what I have read in the bug report thread about him, it seems that ALL the ghost pets that roam the cemetery in Brindleton Bay have been resurrected for some people.
So if yours is a ghost again, I would love to know how that happened... unless maybe your save is so old, that he has subsequently died again of old age?