Need boyfriend advice! girls, what would you do? boys, what SHOULD i do?

basically, my boyfriends ex told me that something 'went on' between them that was really serious, and tahts she didnt want to tell me because it isnt her business. so i asked my boyfriend and he told me that it was that while they were going out, he got her pregnant. she got rid of it obviously, but ive been with him for 4 months and he hasnt bothered to tell me. he wont shut up saying how sorry he is and said he didnt want to tell me because its something in his past that he regrets. all were doing is arguing at the moment and all hes saying is sorry and how much he loves me. he always said that im his first true love and with her it was all about , but this has made me feel thats a lie. but what the hell do i do, this is really serious and i dont know what to do? ive only just started trusting him properly and now this has happened, i mean its obvious that his ex was trying to stir things but what am i supposed to do?