Applying a little imagination to this scenario gets interesting. WHY didn't he title it, and how couldn't he title it?

Didn't change title because he wouldn't be legally responsible if he wrecked it, leaving financial responsibility YOUR problem? In Californication, no insurance, no title, so he never insured it and ran on expired tags, again YOUR problem if he creamed somebody and killed a few people.

He had a wad of cash to pay for the car, no paper trail for purchase, under the legal radar and then didn't title & register: SUSPICIOUS behavior!

Only used car for a short time then ditched it, at an airport: Drug courier, using YOUR car titled in YOUR name for deliveries? Made his nut on drug sales, then hopped a plane back across the border?

Was already having 'Legal problems' when he bought it, so never registered it, then finally sky up and bail out to stay ahead of the law?

Wasn't a legal citizen so no proper ID to register it, no driver's license, no insurance, use it for a while and hop a plane back home?

Might want to check trunk, innder fenders etc. for any new compartments, or any white powder residues, or anything that looks & smells sorta like really potent alfalfa. I'd sit on it only after I made sure it was clean, and after talking to John Law to see if there was any 'Wanted' or 'Person of Interest' on the car or see if they want to swing by with a drug sniffer dog.

After that I'd hold it for a reasonable time, maybe a couple of more months. He would certainly owe you all fees, plus a reasonable amount for storage daily at your place, if you wanted to charge for that too.

Regardless he owes you some grief & misery money, but the main thing is he could leave you legally responsible for his actions once again if you surrender it to him! Obvious he can't be trusted for whatever reason. I see no big moral dilemna on your end; he's the one that flaked and left you to take the heat if he wrecked, and it's not your fault he didn't get around to title and registration in his name for over a year!