As the title says, my Younger brother (Turned 21 a month prior to this) was gifted a handgun by our mother's boyfriend, a Sig Saur P322 9mm, rather old. A problem has arose, however. My mother is looking to break it off with this guy (He is a dick who honestly is one of those few people you wouldn't trust with a gun, how he got his I will never know.) My brother, and myself, are worried that this man will try and take the gun back, going so far as to call the cops and say we stole it. We do have 3 witnesses to saying, 'Hey, he gave my brother the gun, it's his, right?' but we are looking, regardless, to get it in my brother's name in the eyes of the Law (With him being military and whatnot, he kinda wants to stay kosher.) What exactly is the process for this? Is there even a process at all?

I am sure this question has been asked often, I just can't find it anywhere on these forums. Any help is greatly appreciated. Thank you!