Dreaming about your boyfriend with someone else is a common dream. It indicates that while you trust him in real life but really inside you don’t trust him to be alone with any female friends. Maybe in the past he has cheated on you before, and any answer he tells you becomes a bigger lies that turns into recurring bad dreams.
The only thing you can do to keep a close eye on him, while giving him plenty of space to prove you wrong. This will help you regain your own sense of confidence and security in the relationship.