I agree with what Anvil said above, you owe nobody anything here. Take time to think about what feels "right" for you, and then go with that. You are under no obligation to be a "conveyor of messages" to and from jail or anywhere, for anyone.
If you do choose to respond, then I like Laurie's suggestion...
"I will be happy to pass on your message and if he wants to contact you he will."
I am selective about discussing my son with people, and I no longer squirm when I say "I am not comfortable discussing this" and change the subject.
Hugs