You can load the tablet with books yourself and then send it to him. I presume his parents have a computer? You can continue to add books by sending him a flashdrive through the mail with more books that you have purchased and that can be sideloaded on his device.
Many parents simply shut off purchases for their kids and side load all content. Lets them control what the kid is doing and prevents their credit card info from being stolen by a gifted hack among his friends (or their older bro or sis).