KIlroyJ,
There is no such thing as a "must" event. The Series Classic is a newer event that is highly selective. In a perfect world we would have the very best players from various areas of the country at their age group. Of course, we know that isn't always possible. Especially in the younger age divisions.
It is true that the better your son performs the more this event and other events might help him. However there are some events that simply being on the roster can be beneficial. PG National and Junior National, PG Select Festival, PG All American Classic, Area Codes, ECP, USA Baseball National Team, etc. These are places where being on a roster means he is considered a top player. That is because of past results show that those type events are always full of highest level DI players, early draft picks and future Major League stars.
Whether you decide to go or not, if your son was invited it means we believe he is very talented. Never feel like there is something you absolutely "must" do. But if he has the talent do make sure that he gets nationally identified as much as possible. It will create more and better opportunities.