Need advice!

Ive have been with my boyfriend for 4 years now.. He has never really had trust in me because as he says its because i dont want to change my attitude and the way that i act towards him. Long story short one of his friends told him that they have seen a guy around my house, when noone has ever came to my house for one because its my mom's house and theres no company allowed but two im in a comitted relationship and would never cheat. Anyways, ive been going through that and now i can possibly be pregnant and now hes saying it would not be his!! Im so angry! Because he said if im pregnant hes leaving me when i know it would be his and ive been trying and waiting for this moment for months! So what do i do if he really does leave me, give it everything up for him or let him leave and have our blessing that he doesnt think is his.