I'm not sure this is legitimate yet but wondered once I wrote him a letter using this site, would he need to be prompted by me on how to respond?

What jumps out at you that this might not be legitimate? There are over 300,000 pages of content in the site synced to 17,000 prisons in the United States. InmateAid's service is not email; it is with the US Postal Service. Inmates do not have Internet access. We estimate that it takes 2-3 business days to make it to the jail. Once there, the staff opens and reads each piece of mail and inspects it for contraband. Once they decide the mail is fit to be handed out at mail call, your inmate will receive it. Any delay that occurs at the facility is out of our control. We make no guarantees as to how long it will take as there are thousands of facilities and none of them do things the same way. Please be patient, if the ID number and facility are correct we are 100% reliable. Inmates that have money on their inmate trust accounts can purchase stamps and envelopes at the weekly commissary. If they do not have money on their books, the prison will provide indigent inmates with all the materials necessary to send out mail to their loved ones. If your inmate writes to you directly, using your address, the cost of the mailing is a 49 cent stamp. Many of our members use the Inmate Response Mail service through InmateAid. Your inmate would write you back to the return address on the letters which are to the InmateAid corporate offices in Florida. We receive the letters on your behalf, scan them into your My Account section and notify you by email that they are waiting for your review. The people that use this service like the security and privacy this service provides them.