Do you have a bill of sale? I am not a lawyer, however I worked in the insurance industry in Michigan so have some knowledge of motor vehicle code.
Whenever I sell a car, I always meet the seller at the Sec of State (DMV) and finalize everything there and then. That gets the car out of my name and I am not liable if he goes out and drives it into a school. With no bill of sale and without him retitling it in his name, you may be liable if something happens when he or anyone else is driving it.
My first stop would be to your DMV. You could force him to retitle it in his name by going to the DMV and applying for a duplicate title saying you lost yours. This is not the right way to do things and I am not advising you to do that as you may get in trouble. However, if it went through without him contesting, you could theoretically try to repo the car as his title should be null and void now. Again, not advising you to do that.
Some states have something called a Transfer Notification that you would fill out when you sell a car. Not sure what it does exactly, but it may have been created to protect sellers in exactly this situation.
It never sat well with me to sign over a title to someone and just hope they retitled it like they are supposed to do. If they didn't and the buyer got in trouble, believe me, the insurance companies would be on you right away for damages. You could reasonably be held liable for those damages as you still are titled on the vehicle.
That would be my biggest concern right now, liability. If he ran over a bunch of nuns (God forbid...) or caused a train derailment with "your" car, it would get messy and I've seen the insurance companies go after people for really big sums of money. If I was the insurance company, I would DEFINITLY go after you and force you to prove you did not own the car, which you may be unable to do because it's still titled in your name.
All states have laws as to how long an individual has to transfer title. Check with the state DMV.
D