I like this thread. Welcome LKM!
All valid points here. One thing to add may be: Expose him to a very wide variety of activities and locations. If you chose to acquiesce to his mother's team football ban, play it just the 3 of you in the park. Take dance lessons (from video??) and practice in the living room. Sing. Play/make music. Go for walks and hikes - without stopping to preach to everyone you see! Have neighbors over. Plant a little garden. Learn to play backgammon or chess. [Although the mom may object to chess. The WTS has had problems with it in the past. But give it a try anyway.]
In contrast, his JW community will likely only have 2 or 3 activites to do with him: Meetings, FieldService, DonutStop. Social activities will probably only happen 2-4 times per year.
The main thing is that by exposing him to an increasingly WIDE variety of activities (both mind and body), people (all ages - just like at the KingdomHall; AND seek out friends from school his own age), and belief systems, you will help prepare him to make his own decisions as he grows older - whether in or out of the JW 'cult'.
Looking forward to future posts from you.
-Aude Sapere (meaning: Dare to Know; Dare to Have Wisdom/Understanding; Dare to Think for Yourself)