I assume you were not married...in which case AFAIK...
The old car was yours, you sold it and presumably pocketed the money, although its irrelevant whether you pocketed it or not...so long as you received the money for it and did with it what you want.
Regarding the new car: It was left to him in a will, so its his; he may of agreed to keep it and allow you to use it, for the families benefit... but its still his. So unless there is any documentation to contrary, (eg a signed agreement that he has gifted the vehicle to you) then you will have to give it back to him or buy it off him....perhaps with the money you received from the old car?
If you were married - you own everything!
Hope this helps - glad you are out of the relationship. Good luck