Hi Momma-
I could be way off, but your stepson, seeing you as mommy first, may feel that you have "let" him get so far away, and his reaction is anger towards you. He may also miss his dad and because he doesn't get to see him that much, may be angry at you for being the stepmom because now he can't spend time with his parents all of the time. Really, it is not your fault in any way, and his emotions are hitting a changing point with puberty as well.
I reccommend you tell him how much you love him, how much his dad loves him, how much his bio mom loves him and do a little pushing back of your own in love. It takes a long time for kids to come around in certain situations. Just don't let the bond break- make a point of calling him, inviting him places, etc. Expect of him, when he is at your home, to be involved in ALL family activities, and maybe plan family game nights for when he is there. Board games are not a thing of the past. Pictionary, catch phrase, etc. These are really fun and really easy games. Cranium is good, too. Anyway, just envelop him with all the family love and appreciation as your son (step son) that you can imagine.
Good luck momma
-E.