I'm going to ask my partner not to go out with his work friends
Why don't you want your partner to go out with his work friends?
I'm going to forbid him from going out with his friends because I don't trust him.
You should not try to control your partners actions when you are in a relationship. Instead, you should understand that he has to have a life outside of your relationship in order to be completely happy. If you try to stop him from hanging out with his friends, he may begin to resent the relationship he has with you.
I bet he's going to cheat if he goes out with his work friends.
It's good that you're aware of your feelings; however, exerting so much control over your partner is unhealthy. You need to find out why you're feeling so uncertain about him. If he's done nothing to make betray your trust, you should consider couples counseling.