I'm going to ask my boyfriend to ditch one of his friends
What is your issue with one of his friends?
He has been stealing my boyfriend away from me.
I do not think that you can ask him to do that. You should not dictate who they can and cannot hang out with.
We do not like each other.
Telling your partner who they can and who they can't hang out with is not a great idea. Feeling restricted in your own relationship is very problematic, and can lead to issues down the road for you guys. You are better off just focusing on being the best partner you can be instead.