My two cents worth says you did a great thing for him. He gets the best of all worlds to enjoy the outside with safety. He obviously loves it. I don't know if you can just ignore the yoweling during those times when he cannot go out, or not.
We have a slightly similar situation. We now live where we can let our previously indoor cats out into a special cat fenced yard (they can't go out in our yard and prey animals can't get it). The cats love it and yowel when they can't go out as well. There are times when we don't want them out and we just ignore the yoweling at the door...and it eventually stops. They move on to other things inside. Could you try bribing him away with food or toys or hold him to distract him and maybe he will give up for awhile?
Otherwise I would keep taking him for walks since he loves it. Seniors deserve some fun, don't you think! Congratulations on such a long lived kitty!