I don't know if i should trust him anymore :(?

me and my boyfriend nathan have been going out for 4 months, but we've been talking for a year, when we first met, he was going out with a girl named stephanie, while they were dating he told me he liked me, but went out with a girl named ally after stephanie broke up with him, he told me he asked her out because he felt bad for her, then a week later he broke up with her and was picking between me, Kellsie Nichol, and kelsey fox. he told me he really liked me and stuff, then a few days later he asked kelsey out, because he was scared i was going to 'hook up' at a party i was going to. while he was dating kelsey he kissed me, and then a day later i saw him at Kellsie's house, trying to kiss her, i got mad and everything, then when we started going out we fought, like a normal couple, we've shared things with each other and hes never been this close to some one, he trusts me, but IKD if i can trust him. 2 nights ago i got a new phone and i was messing with him by saying things like 'hey sexy' and things like that, i was about to tell him it was me when he started flirting back, so i asked if he wanted to have sex and he said only one way to find out, that's if she comes over but he never said he'd do anything and was always trying to figure out who it was. when i told him it was me he freaked out saying he wasnt going to do anything. then today he came to my house crying and playing our song, he says he loves me but i dont know if i can trust him :(