You've submitted a pre-read in good faith. Your son will be submitting an ED application in good faith, yet there are no guarantees by admissions or the coach your son will be admitted. The risk is all on your son, and for this reason I would attend the showcase for the purposes of possible RD application to another school. You are dealing with a process where the outcomes are not always positive.
My son kept in communication with some schools after he committed and applied ED to a D1 HA schools with slots. He fully intended to honor his verbal and ED committement. I suggested to him that we only get one shot at this, and we are taking on all the risk. A reasonable person needs a contingency plan in life and in college baseball recruiting. I strongly encourage your son to attend the showcase.