As I am not licensed in your state, I can only give you some general advice. It sounds like you have made the right decision in making this boyfriend your ex.
I would stand your ground. He has been using your automobile and depreciating it for 2 years. Many rental companies charge $.25-$.35 per mile for depreciation. If he made repairs to the vehicle without you asking him to do that, I don't see how you would be responsible for those costs.
The tickets etc. are not your responsibility. If some of the red light tickets have been issued because of an intersectional camera, you can usually file an affidavit to swear that you were not the driver and you will not be responsible for the tickets, as they cannot place you behind the wheel.
If your ex threatens you, called the police.
If your name is on the title it is your car. People come up with these little fictions that because they had bad credit and they couldn't get the car "in their name" then, that it is really their car but we "put her name on it." I can only guess that this is the sort of fiction that was going on in his head. It is all rubbish.
If your name is on the title, it is your car. (Unless you're married, then it is marital property and disposition would depend on the MD martial laws.)
His claim, if he took this to small claims or another court, would be for "his" (?) down payment and "his" payments periodically for the car. You would counterclaim that he stuck you with $ xxx in tickets and violatoins. Plus, I agree with the previous answer you received as to the value of the mileage on the car so take the number of miles on the odometer when you retrieved YOUR car back and multiply it by, I think, .48 for each mile.
There is a quirky little lawsuit that pertains to real estate, but NOT to personal properties like cars, called a 'petition to partition'. Briefly, those cases involve jointly held property in which the joint owners squabble about who contributed how much to the property, who made improvements, and other factors, and the court then decides who gets how much in the end. That does not apply to a car but that sounds like what your X wants to turn this into. The reason I bring it up is that the basic underlying assumption is that real estate generally goes up in value over time, or at the very least retains value, and that is generally true, despite our current downtrend at the moment. However, anyone smarter than a fifth grader knows that a car depreciates immediately the moment it is driven off the dealer's lot.
So, the idea of getting back payments on a depreciating investment is spurious at best. Payments on a car are really not an investment in the sense that anyone knows they will truly receive a significant return at any time. Payments on a car are more in the nature of payments for use of the vehicle, which we all know will go down in value and ultimately go to car heaven some day unless we have the good sense to trade it in before the final major catastrophic event occurs.
Two more quick legal theories:
It was your car and he may have gifted you the payments. A gift is irrevocable, meaning you have no obligation to give it back. Whether this legal theory flies in your case depends on what the nature of your agreement was.
Finally, he seems to be saying you had at the very least an implied contract as to the use of the car and the payments for the car. Whether it was a contract, an implied contract, or a case in which he's claiming he conferred a benefit, without a contract, for which he should be paid (the old law calls this "Quantum Meruit") you might assert all of the above defenses plus illegality. Where was the car titled and where was it actually garaged every night? Whose name was on the title, yet who actually was in regular and substantial control of the vehicle? Did he have your consent to use the vehicle at all times? Where did you tell the insurance company the car was garaged every night, and where was it? There are a lot more questions, the answers to which could build a defense that the arrangement was void for illegality.
So, "can he sue you" yes of course he can, but you have valid defenses.
Sign up to receive a 3-part series of useful information and advice about personal injury law.