No, responding to a letter does not cost him anything beyond a postage stamp, which is either provided by the facility or purchased through commissary for a small amount. That is the same cost he would incur sending any letter to anyone on the outside, and it has nothing to do with InmateAid specifically.
When you send a letter through InmateAid, the cost is covered entirely on your end when you place the order. The letter arrives at the facility as physical mail, goes through the standard mailroom inspection, and gets handed to him at mail call at no charge to his account.
To write back, he simply addresses a letter to the InmateAid return address printed on the envelope you sent him. That address is in Florida and appears on every piece of outgoing InmateAid mail. He writes his response, puts it in an envelope with a stamp, and mails it the same way he would send any other letter.
When that response arrives at InmateAid it gets scanned and uploaded to your account dashboard. You receive an email notification letting you know a letter is waiting, and you unlock it for $1.59 to read it. That small fee is on your end, not his.
So the financial picture is clean. You pay to send. You pay a modest fee to receive his response. He pays nothing beyond a stamp. His commissary account is not touched by any part of the InmateAid correspondence process.
Go ahead and write. There is nothing on his end that would prevent him from writing back when he is ready.