My boyfriend lied to me about a one night thing

Hey, Basically me and my boyfriend have been together for 4 months and over the past week people have been saying that my boyfriend cheated on me and that he has been going around telling people about it but saying that he spoke to me and we sorted it all out. When I asked him he said that he had no idea what was going on and then told me he had spoken to the people that were saying it and that I didn't need to worry about it. This made me worry because I wasn't sure why people would say it and then why he would tell me to ignore it all. So I went and spoke to his best friends (he gave me their numbers knowing that I was going to speak to them about what was going on) and they said that there was truth behind the rumours. So I confronted him and he still lied. I forwarded the texts from his friends and then he started telling me more and more about the truth (only doing it because there was no point in lying to me about what I knew). We met up yesterday to talk about the situation and our relationship. He told me that two months ago he was at a party and a girl tried it on with him but he told her to leave him alone. Then he fell asleep and he claims that she did things to him when he was asleep and he was told three days later. I'm not sure if I believe that because he often has girls try it on with him at parties and he tells me the next morning and we have a laugh about it, but this is the first I'm even hearing about this party. I have told him that he is never to see this girl again and that we are on a break at the moment, so this is his chance to prove to me that I should trust him. I have also said that if he gives me any reason to not trust him then he will not be hearing from me again. I think he is genuinely sorry that he didn't tell me and that all of this has happened and he really does want to earn my trust and respect back. Sorry that this is such a long post, but the only way that you guys will be able to understand my questions fully is if I tell you as much as I can. So what I'm asking is, should I see this as cheating? I mean, he could have left if he wanted to. And should I believe that he didn't know what had happened? Also, anyone who has been through the situation or knows what it is like from either point of view, how can he show me that I should trust him again. And how will I know that this situation is past us? He is 21 and I am 17 btw.

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