Okay so i just need to get this out : My SO is from another state he moved here for a job a while…

Okay so i just need to get this out : My SO is from another state he moved here for a job a while ago , before me he was with someone else and they had children , we aren't going to say how many BC its a lot lol but anyway whenever he talks to the kids they get on the phone asking his name , he'll say daddy , & they reply with " who Maurice ? " That's not my SO name im literally in tears , why is this woman having them kids call someone else dad just because he isn't in the same state & the guy she's dating is in prison like wtf. My SO is an awesome dad & takes great care of his kids and it hurts me so much to see him like this , he's broken and idk what to do ..