I wasn't a step, but was 1 year younger than my cousin. Whenever we came to town everyone made me go out with my cousin. She made my life miserable in weird little ways that the adults couldn't see, and that I was too young to know how to point out. We would have gotten along fine enough probably, except that she felt "stuck" with me when she wanted to go out with friends or boys (her family was more "liberal" than mine, and her mom would make her have rules when I was around so as not to look bad to our grandparents). She would make sure to let me know there was a party she was missing, or some way my presence was ruining her life, and I felt bad about myself more than her situation (until I was 16 and able to handle myself better, lol).
It's important to know the age difference for the stepson/son. If they're close in age, I would suggest asking your son if the stepson could go with his friends and see how he reacts. Tell him that his stepbro is family and a guest, and we don't want to make anyone feel left out, and if he is cool about being a good host, you can throw a simple party for the friends they'd be trick or treating with (before or after) with snacks, orange or green punch, an appropriate Halloween themed move, take pictures and have fun with it. If your son bucks the idea (or the age difference is too great), I would let your son go on out with his friends (no guilt) and then have a family party at home for the stepson: let him help you make popcorn balls or caramel apples, (or at least Halloween cookies), and make it a time that you get to spend quality time just with him, like it was planned that way because he's special. After time in the kitchen I'd probably dress up silly, and take him trick or treating. Other fun memories I have of Halloween include roasting marshmellows over the grill, watching Its the Great Pumpkin, Charlie Brown if real young, if a little older take turns telling scary stories. When your son returns, of course let him join in on eating the treats you guys made. Have fun regardless!