While he is still this angry, and this strong headed on the issue, the best thing to do is to let it go. Yes, hes said hurtful things, but more then anything else you *have* to be the bigger person here and let him vent and try not to get too upset.
Prove to him that its a safe topic by being happy in your choices and being happy with your life. If he sees you happy its pretty much the only thing that will counter the bullshit thats going through his head. This is very very difficult, but trying to talk to him now, more will probably only result in more hurtful things being said to you by him. Worse, it might result in him or you making an emotionally charged decision based on the forced conversation that will make things much worse between you both. he dosent want to talk about it: give him that space. then see what happens in a few months.
As for forbidding your step mother to mee the bf: she probably has to stand by him or face punishment / wrath from your father as well, if hes anything like my dad. But at some point she will make her own decision about that. id ask her what to do about it, she probably knows him and his reactions better. But anything more coming from you only risks inflaming the situation further.
Let some of the anger die down. try and just let him see you be happy, and see if its possible to go near it again at a later date. Dont make any rash decisions >< i know how much it hurts. Im sorry you have to go through this.