Have you offered to pay him market rate to implement it? If yes and he doesn't want to, fork the project and pay another dev to do so and then see whether he is

Have you offered to pay him market rate to implement it? If yes and he doesn't want to, fork the project and pay another dev to do so and then see whether he is willing to merge the feature. If not, maintain your new fork. If you haven't done the above then you have no right to demand that someone performs work for your benefit for free.