S.C.
Mom, he's just growing up and coming into his own. If this is all of your worries, you really have none! If his grades are good, he's going to school, doing his work, coming home at night to go to bed, etc...then allow him to venture into new territory and explore. Don't fight with him, but you are STILL HIS MOTHER AND WHAT YOU DO NEED TO DO IS DEMAND RESPECT! But you then have to give it too! Your boy is growing up and he will begin to make choices that differ from yours. As long as he is not hurting him or anyone else, your job is advise at this point, not control. Ask him if he would be interested in having some gatherings or parties at his home once in a while.
My son is 26 and my daughter is 13 and yes they fight about things, but he is not mean to her or makes her cry. If he did he would pay the price for it. He knows that would never fly in my house. Maybe you should observe them together a few times without them knowing it and see what's really going on. Otherwise maybe him not being around her so often is better at this stage until he gets past whatever is causing him to do this. You also might video tape him with her one day and then play it back for him. BUT YOU HAVE TO MAKE SURE THAT HE FEELS AS LOVED AS YOU DEFENDING HIS SISTER. Let him know the very uncomfortable position he is putting his mother thru and try to always paint a picture "if you were in my shoes" and maybe he will see things a little differently. Also BE VERY CAREFUL OF PRE-JUDGING AND JUDGING HIS FRIENDS, HIS LIKES AND HIS DISLIKES! Remember he's your child growing, not a possession to do exactly as he's told anymore. Next year, he could leave you officially, if he's not happy! You do not walk on pins and needles with him however and always demand respect, but make sure he knows you expect him to be treated with the exact same amount of respect! Let go of all your rules and JUST TALK WITH YOUR SON FOR REAL! Where is his REAL FATHER? This is something you might want to consider, him getting to know his dad if that is possible. Trust me, my brother was adopted when he was just a baby, but my parents had to deal with all the issues of identity and the "why's" they ask themselves of them being put up for adoption or their daddy leaving them. Good Luck