Hello again, (I wrote this before I saw your last post. read it and let me know what you think)
Sorry you find yourself in this dilemma. I am afraid that the resentment that bf will feel is going to make things worse between bf and son. Even if it is unconscious, bf is likely to project his anger onto son. If you have long term plans for this relationships, you may need to work this out in a different way.
It is not unusual for a child to resent and dislike the "step-parent". There is nothing you can do about this and moving bf physically will not change things. They need to learn how to co-exist if bf will be in his life from here on. If son threatens to move to father, call his bluff. He might go but will find out the "grass is not greener". People who live together need to find ways to co-exist.
For one thing, bf should not discipline son and confrontations should be avoided at all costs. This will need to be the compromise that bf offers. He is the adult and when confronted, he should say, "I can see you are upset right now...let's talk about this later when we are both calmer or let's get mother in on this discussion." In addition, if a house rule is being broken, bf should not enforce rule but rather tell you about it and be reassured that you will enforce it. You may want to have a family meeting and let everyone know that this is how discipline will be handled. You and bf need to be on the same page regarding the rules and the consequences. If this is difficult for the two of you, seek counseling to help you.
Here is a website to assist in finding a therapist to help:http://therapists.psychologytoday.com/rms/
I hope this different perspective helps.....warm regards,