I'm worried for my child's future and so I've been sabotaging his friendships.
Why ruin his friendships? Why are you so worried about your child's future?
His friends are a bad influence on him.
You shouldn't sabotage their friendship because of fear of disloyalty. If they find out that you are sabotaging them they will hate you and never talk to you again.
I get so tired of their disloyalty and if they hate me and never talk to me again then so be it.
Who your son hangs around with is partially your business as well as theirs. Ask your son if they like the friends they have and try to understand why. You should want your son to enjoy their friendships not destroy them.