Our step son has slept with his mom for the past 4 years. We are getting married in a month..Now what do I do?

She tells me everything will be fine after we are married and I disagree. We have tried numerous different gifts to entice him to sleeps in his room (bunk beds, cell phone) but he still insists on sleeping in his moms bed with her. Her son and I get along great and I love him as my own child but there is a rivallry between us as it seems. If I hug my fiance he will run up and say "my mommy" and give her a big hug. Now after we get married and we move in together I plan on sleeping in OUR bed with my wife. But I don't want him to resent me because he can't sleep with mommy anymore! I am just not understanding how in 30 days it is just going to change because we are married. By the way to make this even more interesting our son is **** 9 **** almost 10