Yes, I think the best thing is to legally gift him the gun.
If you want to call it a loan between the two of you then he can always gift it back to you.
the big deal will be whether Mass. requires you do a legal transfer for this with an FFL. (I don't have a clue on this) most states don't but Mass might... but it is usually a token amount.... 25-50 bucks.
I didn't see different but assume you both live in the same state?