PLEASE HELP!! My boyfriend is too paranoid to have sex with me.

Me (16) and my boyfriend (17) have been dating for almost 2 years, we started having sex about 6 months ago, my boyfriend is extremely against having children and never wants any. he says that he enjoys sex, every time we have sex we are extremely careful, we use a condom and he doesn't finish in me even with the condom on, but he remains paranoid every month that I am pregnant, which is practically impossible. Last month we had more sex than we've ever had in our relationship and my period was 3 weeks late, he panicked and almost even bought me a pregnancy test, I enjoy sex and want it quite often but he said he will try to avoid having sex with me at all costs because he is concerned that I am pregnant. How do I assure him that he’s overreacting without getting on birth control?