Gretel, I feel for you, and can see the difficulties involved in retiring if you have such large expenditures for his flying. I think the solution might be found by looking for middle ground. Maybe look at ways he could fly less, but continue to fly, if that makes sense. I would ask the following:
How often does he fly? If he sold his stake in the plane (expensive to own and pay for maintenance, etc) how much cash would that free up for renting a plane to go flying? Could he sell his stake and fly less often?
Although I see what people are saying about his hobby being selfish as it is so expensive, but can also see Gretel's point about him loving it so much. I think some hobbies could be better described as passions, it seems flying, horse riding, motorcycle riding, all come under the latter - more about a way of life than a part time fun thing to do. If he gave it up, he might come to resent his wife and his retirement, which would hardly make for a happy life, would it?