I think both of you have some facts muddied. Unless he bought a trailer, there are no "paper tags". There's no such thing for a boat. Google "TPWD Boat Title" and read. It's unclear what to tell you to do as I don't have a clue what paperwork you actually have. I forget how Academy handles those sales now. There may actually be a title he received in the mail from TPWD some time after the purchase. If he doesn't understand boats/titles he may have tossed it as a junkmail item as it is a simple perforated form folded into a letter.
You can look up the serial number of the boat on TPWD to see if they have a title for it. They'll list the owner's name if they do. If so, he'll have to get a replacement title, then agree to transfer that to you. BTW, you should never buy a boat in Texas without getting a signed-off title along with it. Without that, you haven't really bought it legally.