My bf wants another baby, and we haven't even had our first yet? ?

I'm currently 26 almost 27 weeks and the baby really seems to excite my bf, who's 18, and it's really weird. We haven't had our first one yet but he wants another one!!!! Is it jut a phase? Or what? How can I wt him ot of it! I'll be 18 next month and don't want another baby because I would like to go to university.. But he wants another one :/ me being pregnant brought us closer together and we learned new things about each other. Maybe he thinks we won't have that bond later? I don't know what to do... Help!!