You can say that you were researching it with your religion and found out that it's against it. But if there are other laws in your are (such as personal beliefs) then I would go with that.
I try to be honest in all areas of my life so I can understand if you don't want to fib about the religion thing. If your country/state doesn't allow anything but medical or religious exemptions, then you may be forced to fib for the safety of your child.
If DH wasn't with you at the last visit you can always explain that you had changed religions when you married and weren't sure of some of the nuances, you're still learning yourself.
If your state allows personal beliefs, then I would go with that one and not give details. Just keep reiterating that it's a deeply held belief. As he offers stats and reasons why you should, smile, thank him, and hold your ground peacefully.
Stay away from the medical debate, it's not worth the hassle as doctors are indoctrinated to believe a certain way and they feel they are doing the greater good for children by vaccinating.
We never went to a WBV after my son was 2 weeks old. It wasn't worth the hassles, YKWIM?