Rachael B gave you some good advice on letting him fall... but before you do, make sure he is 18 and legally able to be on his own or his father might be held responsible if anything happened to him on his own.
I know that age well... teenage years are the worse. They think they are grown up but they don't realize that being grown up is by being responsible, more then age. You might sit down with your step son and tell him when he graduates he may move out, if he has a plan on how to support himself, a place he is paying to live, no bumming off friends, and ask him how he is going to clean and cook and do his laundry if he isn't willing to learn it at home before moving out. If he is reasonable at all he would see the truth in that... but he is 17 and reason doesn't fit into the picture. My son moved out as soon as he was 18. When he had to pay his way, his tune changed big time.
Now for my philosphy on teen years... Aliens clone our good children and take them, leaving us with this undesirable clone! They keep them a few years and then return them to us.
Words to remember... This too shall pass! Before you know it he will be working hard, living on his own, getting married and having kids in which he will say to you "I can't believe he thought he could get away with that! Doesn't kids have any brains these days??!!" With that you just smile and say "Yes, I know, I had a 17 year old boy at one time"
Good luck and remember even the animals have trouble with their grown sons... fighting dad for the top place in the herd.