If it is a sex only relationship, he may leave you if he hears you are pregnant. The only way to really find out how he would react is by actually telling him. I have a friend who was in the same situation (and 13 years old). She had been with the same guy for two years, but only for intimacy. It suprised her (and me) when he stayed by her side upon finding out she was pregnant. You should really talk to this guy and see what he thinks. You won’t know unless you ask. You should also take a HPT or go to a clinic to make sure you are pregnant.
Hope that helps!
Maui