Hi Sabrina and welcome.
I have been DF'd for 20 or so years now and my mom is, and always will be, a JW. I have two sons, now ages 19 and 23. When they were small, she used to talk to them about the 'truth' at every opportunity and drag them to meetings whenever possible. They always came home and talked about Grandma's 'crazy religion'. They were never confused about it and never really asked questions about why I never went to the Kingdom Hall. They just knew their mommy wasn't 'one of those' and knew they didn't want to be one either. But, despite this, they loved, and still love, their Grandma.
I wouldn't worry about it and be thankful your mom still wants to be a part of his life. So many people on this board experience the opposite.
Take care,
GH