Boyfriend problem really need help ASAP please?

so my boyfriend and i agreed that he wont go out unless i approve and now i found out he's going out with a group of girls and he already approved and everything when i told him not to go he said he will and we kept arguing and then he said he's going and that's finale .. and there will be like 10 girls and like 7 guys and his ex will be there and he cheated before so i don't know what to do ?? how to get him not to go ??? .. and should i break up with him if he did ?? .. please tell me what to do (( btw i can go out with him but i will look weak because it will be my last option and i don't wanna look weak in front of him because he cheated before and i forgave him easily so i don't want him to think its ok )) please help :( :(