Hi Bullcat
If I understand correctly, you have a /24 segment for your external IPs and your customer wants to only allow /28 to his whitelist. Now I am making the assumption that there is no routing being exchanged between both of you. If he doesn't mind the extra management needed for that to work, then he can put all the /28 that make up your /24. As far as you are concerned, there is nothing you need to do on your side.
If there is routing being exchanged between both of you, then that is another matter. If he can or wants to only accept /28, then you will need to send him your segment in /28 incrementations for him to accept them. But again, this is only for routing.
Good luck