As a pokedad myself, I can sympathize with your frustration. The system doesn't work for everyone -- not everyone has the time or money to travel around their region to enough tournaments to qualify through championship points, and Nats is a bit of a crap shoot (although I agree that no one lucks their way into the T8 of Nats). However, I think you need to regain a little perspective:
1) Worlds is not intended to be accessible to everyone as competitors. It is intended to be a reward for those players who play a lot, at a consistently high level, and those who reach the late elimination rounds of nationals. There are a lot of really good players, including players who have reached T8 or better at prior Worlds, who did not qualify this year. Could they have been successful at Worlds had they qualified? Probably. But, they didn't qualify, and that's just the way it is.
2) That being said, Worlds is set up so that attending, even if not as a competitor, is a worthwhile experience. Aside from the pagentry of the event, there are a lot of side events and the pre-release for the newest set that are accessible to everyone. The Grinder, while giving players a last chance to get into the main event and Pokemon an opportunity to fill in the field, is more importantly another aspect of the Worlds experience Pokemon gives to non-qualifiers, and therefore it absolutely belongs coupled with the Worlds tournament.
3) US players already enjoy a significant advantage over non-US players when it comes to the Grinder. Although there is an additional financial burden when Worlds is in Hawaii, it is in Hawaii at most every 2 years. The rest of the time, it is in the Continental US and pretty easily accessible for anyone who wants to go. European players, Japanese players, South American players, etc., never have that luxury. Add to that that Japanese players have a much more limited opportunity to qualify for Worlds in the first place, and it seems a little difficult to complain that we have it rough.
4) Finally, the people who staff these tournaments generally have lives outside of Pokemon. They already run tournaments from BRs in September through Nats in June/July. Expecting them to run another large tournament in between Nats and Worlds is asking a lot. Those people already are going to be a Worlds, so running the Grinder at that time is simply far more convenient, which it should be. After all, without the TOs and judges who run these tournements, Pokemon cannot exist, and we should be striving to make their work easier, not harder.
In the end, it sucks that your son didn't qualify for Worlds. Neither did mine, and hundreds (if not thousands) of others. However, Pokemon provides an activity that I think is worthwhile for any kid, and can be enjoyed at a fairly low cost throughout the year. That Pokemon makes Worlds -- which by design is intended for a limited field -- less accessible, hardly seems unreasonable.
If your son really wants to go to Worlds, and you were prepared to take him if he qualified, you should go. He may strike gold and qualify but, even if he doesn't, it should still be a great opportunity to mingle with players from all over the world. And, that's an opportunity he might never get if the Grinder were decoupled from the main event.