Hi R.,
Boy, that is a tough egg shell situation. I'm sure the stepson is feeling off-kilter, and you are between a rock and a hard place, just wanting peace and things to work!!
My first suggestion is to get a copy of Liberated Parents, Liberated Children by Faber and Mazlich.
I've read shelf after shelf of parenting and psych books, and this book turned my whole life around. I was a special ed teacher and the difference I saw was staggering and just beautiful.
They have a workbook, also suggest, How To Talk So Your Children Will Listen, and How To Listen So Your Children Will Talk.
It is based on the work of Dr. Haim Ginot, who wrote many books, also wonderful, such as Between Parent And Child.
For now, my take would be first that allowing someone to disrespect you sets them up to feel disrespectful towards themselves. It isn't good role modeling, in other words.
You know that, I'm sure...but so often parents and step parents get this "I'll show you" attitude, and I think a more gentle approach goes further.
You are strong enough to handle it, but you are dealing with it because of your compassion for your step son, in other words.
I would sit down with him and talk about how it was hurtful, how you don't allow people to treat you with disrespect, and ask him what was going on. List his good qualities and assure him that you know this is unusual for him, so you really want to look at what was going on...write things down...talk about how this is a hard time for the family, and that feelings can be strong.
Then ask him for other ways he might have handled the situation, what might he have said differently? Where are your boundaries, what do you need? What does he need?
"We are family. We hold each others' hearts in our hands."
If it happens again, I usually say things like "I don't allow people to treat me with disrespect." Walk away, whatever feels right. Make it about you, not him.
That way you are modeling that if anyone is ever disrespectful of him, he can take care of himself without attacking.
One powerful technique I found in those books is if it gets to boiling point, which I pray it doesn't...to say, "I feel very angry right now and I don't want to hurt you. You need to leave NOW."
That's last resort level and has to be honest. I've used that once, and, boy, it really changed a negative situation fast.
Hope I don't sound know it allish. I sure don't. It's just those books....
May you find peace,
P.