Reverse DNS (PTR Record) Request

Hello Sirs,

Sorry in advance if this is not the appropriate place to ask this question. I am helping my son set up his new mail server and he needs a reverse DNS for his domain. He cannot do this on his end since it is an ISP only thing. I would like to know if it is possible contacting anyone that can help us set it up. And of course, will there be any additional costs creating one?

Thank you