Wow - this is a very delicate situation so I think your concern is totally justified. I was actually in a similar situation on the "mom" side with my ex-husband. In the case of my ex-husband and me there weren't all of the issues of religion and driver's license (more about that later) - it was just a case of him wanting power and control and he wanted revenge. My situation was somewhat different in that my ex-husband was abusive and since you have other issues I'm not suggesting that is your husband's motivation.
So here's what happened in my situation:
I let my sons make their own choice without any argument from me when they told me their dad wanted them to move in with him. I told them they were 18 and old enough to make their own choices about where they lived. After a heavy negative campaign from my ex each of them decided to move in with him for awhile. With the older son it was 3 weeks later he asked to move back in with me. The younger one was closer to his dad, so he lived with him for about 5 months before he appeared back on my doorstep asking to move back in. For the younger one this was happening at the end of his senior year in high school and he almost didn't graduate due to the distraction and turmoil this caused him.
I would suggest that you and your husband proceed very cautiously and handle this very low-key. Your step-son doesn't need extra stress and confrontation during his last year of high school. Feeling forced to choose between his parents is really a no-win situation for him.
However, if your step-son talks to you and your husband between now and then expresses the desire to live with you, is very unhappy about his living situation with his mother or doesn't want to be forced to attend her church anymore you could mention that when he turns 18 he is legally able to choose where he lives and that he is welcome in your home. Then let it go. That way he will feel supported but not pressured.
If he chooses to move in with you fine, but I wouldn't have a big meeting with him before his birthday and put him in the uncomfortable position of having to make a choice.
About the driver's license - Be careful what you wish for! When kids get their driver's licenses it can lead to more headaches than you can imagine. Your insurance rates go up for starters, most get a ticket or have an accident (or both) within the first year after they get their license and
if they are late getting back from somewhere it worries you until they are home.
If he does decide to move in with you, depending on the agreement in their divorce, I would definitely see what can be done legally to quit paying child support to the ex-wife for him, but I would be careful that it doesn't appear to the court that the only reason you sought to have him move in with you was to avoid paying child support.
I know you and your husband are trying to do what's best for him, but if it will cause too much trouble for him to suggest it to him when he turns 18 you can always talk to him about moving in with you after he graduates.