What can be done in a relationship to have trust?

I am 17 I have been in a relationship for 4 years since I was only 13. We love each other so much we have the best relationship. We`re best friends, we`re like brother and sister, but lately we have had nooo trust in our relationship at all. In the pass we both had our mistakes but it was soo long ago and after that everything was good, now we just don't have trust. He thinks I have a facebook and I really don't have one, I put that on my lil sister`s life but he still doesn`t believe me and I don`t trust him because I found out his cousin made him a facebook to try to see if his girlfriend was going to attempt to talk to him. He said he has nothing to do with that but he did admit that he knew about the whole thing and that he got on it yeyesterday and looked at 2 different profiles of some random females but what ever I am willing to leave everything behind but he`s not. He doesn`t want to trust me. What can I do? I really want to save this relationship it`s been 4 years and I love him so much. We live together and we almost had a baby when I was 15 and that is what kept us soo in love because its always going to part of our life even tho we don`t have the baby. Please I need help please tell me what can be done!