Ugh this is driving me crazy!

So my baby is getting baptized in a few months. Well 1st of all his dad is pissed cuz I didn't give him the option yes he's apart of his life but only sees him Atleast 2x a week. When I had him he was being a complete ass about everything, so I had decided to give him my last name cuz he's going to find out one way or another. How do I tell him that I gave josh my last name I know he's going to be pissed either way but thag is beside the point. I need to tell him just don't know how to. :( help me please