Early pregnancy

So my serious boyfriend of 4 months (we already live together) has brought up in the past month that he wants to have a baby but has brought it up more recently since his best friend just found out he's going to be a dad again. I personally don't want to have a child until after I'm done with school (which is about 2 years or 5 semesters) but I don't know how to really tell him I'm not ready for a baby...Help please!