Could he, for example, put in a calculated 5% non-voting share in a holding account to be retrieved when he stops flying?
Don't see why not, as long as he has the share it is down to the "owners", I.e. The group, how the group is run, even if you gave him a vote it would only be a 1 in 20 vote the rest of you would have 3 and 1/6 votes each to that one, and I am sure he would agree to abstain anyway.