In my personal experience.......
I think it is about finding a denomination (or non-denomination) that has a pastor/deacon (etc) willing not only to 'work' with him but also willing to accept the stuff he isn't ready to let go of without being condemning.....
Stress that indoctrination is what you teach your child individually (or as a couple) while fellowship is about the associating with those who act as Christians in their deeds and lives. Many denominations for instance not only do charity work but have 'visitation' type ministries should he miss FS. Ultimately most churches regardless of denominational divisions are friendly, it is just about finding the right fit socially.
I disagree with the Christadelphians because he probably would like to keep believing Satan is real.
Depending on your location you might want to see if there are any local Bible Students. Yes the ones who didn't go with Rutherford still exist and are autonomous so their professions do vary as a 'safer-feeling' transition.
This all said......while it is admirable you are willing to compromise, what denomination do you favor?