You probably have to make up your mind that he can't go out on his own, because of his age, medication etc.
After we lost a cat for 3 weeks and the agony of looking for him, we decided to make all our cats indoor cats and to our surprise they all adjusted fine. Even my mother-in-law's cat who had been outdoors for 12 years never asked to go out and at her house he was always outside.
So you could try any of the suggestions from the other posters.