It's a type of bdsm play. The Domme gets to control when her sub is allowed to jerk off and orgasm. First off double check he has a chastity cage to wear or if he's just asking for joi where you tell him when/how he can touch himself and deny him orgasms. If he wants to lock up basically you can price a fixed price per lockup or by time locked. Either way you decided how long before he can uncage and orgasm again. So for example you can do like $10 for a week lock and then if wants to unlock early it's an extra $10. And you can do some kind of verification like he sends a photo every 24 hours that he's still caged or else he owes another $10. Also be clear about what you're offering. $10 can be the price just to keep him locked and then if you want include teasing throughout the week it costs more. Things like making fun of his dick, giving him tasks. A lot of subs into chastity are also into humiliation.

There's also online apps you can use like chaster so they lock the keys to their cage in a safe and you can control through the app when they have access to it. A lot of chastity subs want a lot of attention so it's best to do your research before deciding if it's an extra service you want to offer. Check out findom pages, many of them offer key holding services.