Wanting another kid

So I'm 16 and currently 26 weeks. I'm carrying a beautiful baby boy. We've wanted a girl forever but we're blessed with a boy. When this baby is about maybe 12 or a little older I'd like to try and have a girl. My boyfriend doesn't want anymore kids. I just love the feeling of the baby kicking and of course having an excuse for eating so much food. Lol. But how should I tell my boyfriend and maybe convince him.