It is a gift but there's a lot more to it than that. It is actually a conditional gift - conditioned on marriage. In Massachusetts, with your sets of facts, he is entitled to that ring back if he asks for it.
Tell your attorney to read several cases that are on point: De Cicco v. Barker and Poirier v. Raad.
The law in Mass depends on the intent of the donor and, when someone gives a ring at the time of engagement, it is presume that the gift was conditional upon getting married. So, based on the facts you state, he is legally entitled to get it back. However, how much is it worth? Does he owe you for child support? Does he owe you money for other things? If he owes you money, I would hold on to it.