Keep your foot on the gas.
Did your son go the classic and academic game as a junior? Then you know that that tryout is a meat market, and the senior event may be even more crowded. If you're worried about your son being accepted ED, then go. A coach at a school with no slots will understand your concern. I'd also let other coaches know your kid is gonna be there. Maybe he lights it up and some other school he actually likes takes a shine to him and piques his interest. People have the right to change their minds. We kept at it with both our boys even after the coaches assured us their pre-reads were sam dunks. A pre-read is NOT a guarantee. And for each of my boys, a kid that was in their recruiting group, who they got to know on the campus overnight visit, was NOT admitted.
If you're concerned about your son taking someone else's spot in the tryout, then still go. I don't think your son's being there is going to prevent someone else from trying out.
Also, if you haven't been to the event, beware the 60 yd. dash. It's a slow track.