F.:
I am sorry that you are going through this. I am sure it is hard when you see your grown child suffering.
My advice? Have them enroll in Martial Arts - this will allow them to do something together - that REQUIRES honor and respect. As does Boy Scouts - this will allow them to do things TOGETHER without "home" rules and other people guiding them through an activity or an adventure.
If your son has a hobby that he would like to share with his step-son, have them TRY it together - maybe his step-son wants to feel as if he matters and is still needed. He could be VERY angry that his parents are divorced and now he has not only one but TWO step brother/sisters. And to top it off? Starting puberty!!! OH MY!!!! Lots of testerone going on there!!
DO NOT let your son give up - you didn't give up when he was going through puberty - remind your son of this. It's a VERY trying time - your body is making HUGE changes, brain is going a million miles an hour and girls, well, girls are on the brain. He needs guidance and stability not anger.
I know that is soooo much easier said than done. I need to remind myself this with my boys and they are only 8 and 10. But my daughter is 24 and I remember THOSE times!! URGH!!!!
Bottom line? Marital Arts or Boy Scouts will get them into doing things TOGETHER!!!
My best to you all
Cheryl