So, working within your desire to allow your son to make his own choice for the most part, and not wanting to forbid something and end up with him sneaking around and lying (which I think is commendable, BTW) these are the options that I see.
1. Perhaps you and your DH would rather see your son courting someone so much older and end up marrying her once he is 19 or 20 (that seems to be a typical age in your faith) than see him dating a girl his own age who does not have the same convictions
If so, I think you need to be aware of what you are doing, how it looks, etc and open and honest about that with both your son and this woman )because, as I said above, so far as I can tell they ARE courting whether anyone is calling it that or not).
If you go this route and either does not want to be courting with the other, I think you need to explain that is what this is and limit their time together (as per below) and explain why you are limiting it.
2. If you want your son to only date/court within your faith, but also want that person to be about his age, I feel you need to do everything possible to give him that opportunity:
Try to curtail commentary or trips to WDW or other things that cause him to stand out at church as not of the same faith as everyone else there.
Get him to that church EVERY SINGLE WEEK, even though it is a long drive. If there are youth group meetings on other nights, or anything else at all he can participate in, commit to getting him there.
Also, explain flat out that he cannot be "courting" someone who is 28 if he is looking for someone his age and explain that it IS that whether he perceives it as such or not and then limit his time with his older friend to about once a month and on a family outing of some sort (so she becomes more of a family friend, and so he is compelled to fill that social gap by finding other friends to spend time with).
3. If you are okay with your son dating/courting outside of your faith, let him know that and even encourage it.
Have him join the youth group at your local church, orelse find a different congregation where that is okay for you and him. Get him out and among teens and make sure he knows you will not be upset if he asks a girl with different beliefs out.
Again, limit contact with the friend in the way I said above and for the same reasons.
Anyway, those are really the options that I see for you at this point. The hardest part is going to be being honest with yourselves about what you really want for your son within what is realistic in your situation and then putting it into practice.
Good luck.