Senior dog on 3rd floor apartment

My dog has been getting older (11 years old now) and has recently started having mild arthritis with his back legs. We live on the third floor of an apartment complex and aren't in any position to move, so we're getting worried about his mobility up and down the stairs for daily walks. He's about 65-70 pounds, which I can carry fine, but he doesn't like being carried for so long and it's not a good permanent fix. We don't have any elevators either.

Has anyone had a similar problem and come up with any good solutions? Rehoming isn't out of the question if his quality of life isn't good here any more, but I'd hate that so much. I've had him all his life and he'd be so confused to be suddenly relocated so late in his life.