First of all take the high road.....thankfully the colleges already know who he is...but you need to make sure with his club coaches that him not getting on his high school team has tarnished his image. When talking to the other parents ask them for thier opinion on what they would do....
For me.....I might ask to meet with the hs coach and get his reasoning...but state that you are not here to get him on the team. It's a mute point on that. You'd like to get his perspective on what your son can do to become a better player....if he chose not to meet with you then write a letter to him and copy the principal. But make the letter very positive....and just state the facts......his long time playing club, his want of wanting to play on team last year in hs, other parents surprise that your son did not make it. I am not sure I would mention the interest of colleges wanting him. With all of the facts stated ask if you could get his reasoning on why your son didnt' make the team and what advice he could give your son to make himself a better player....
And now that I am typing this....it might be better for your son to do this himself in person to the coach. Your son needs to go in there and accept with dignity what hte coach has to say whether it is negative or positve....then he can come home and letyou know..and him and the family can process what occurred...this can only make your son a better person.
If this comes out as something that is negative or just plain crazy...write a letter to the principal to make them aware of the situation for future situations and that you are not soliciting a spot for your son on the team.
Sports is tough...but it makes you a better person with the right positive approach. good luck.