What can be done in a relationship to have truss?

i am 17 i have been in a relationship of 4 years sent i was only 13 and my boyfriend is 19 turning 20 and he been with me sent he was 15 we love each other so much we are the most lovely relationship we are each other bestfriend we are like brothers and sister but lately we haven hade nooo trues in or relationship at all in the pass we both mess up in or own ways but it was soo long ago and after that everything was good now we just dont have truss he dont truss me cuz he think i have a facebook and i really dont have one i put that in my lil sis life but he still dont belive me and i dont trust him cuz i found out his cuzzin did him a facebook to try to see if his gurlfriend was gonna talk to him like that and he said he has nuthing to do with that but is hard to belive but he did admit that he gat on it yesturday and he went to 2gurl profile but wat ever i am willin to leave everything behind but his not he dont wanna trust me what can i do i really wanna save this relationship is 4 years and i love him we live together and we did a baby when i was 15 and that is what keep uss soo in love cuz its always gonna be in our minds even that we dont have it cuz i hade a abor...please i need help please you tell me what can be done or if the relationship has no trust there is nuthing to do about it....