He can write you back as long as he has a postage stamp. Even if he has no money on his books, there are programs for indigent inmates where postage is provided.
https://www.inmateaid.com/ask-the-inmate/if-i-pay-to-send-a-letter-can-he-type-me-back-or-does-he.amp#answer
Accepted Answer Date Created: August 07,2016