Yes "preferred walk-on" is yet another slippery term in the recruiting quagmire. It seems that the most common definition is that the student athlete will receive no athletic aid, but has a guaranteed roster spot assuming he works hard, follows team rules, keeps grades up, etc. But here the Coach apparently uses the term differently - no guaranteed roster spot arggghhh....
I admit I am "pro-labor" on these issues LOL, but the letter the student athlete received here should be a good indication that he was not "recruited" - it gives no inducements and promises nothing!! The NCAA of course probably looks at it differently. Curious what Rick's take is. Good luck to OP and his son.