gettinthere -
Sorry to hear about the rough start. Lots of things come to mind when reading your post...
It is great that you are looking for ways to help him through his hitting slump and confidence issues. Fight the urge to help too much or look for immediate quick-fixes. This is one of those great opportunities for your son to figure things out on his own and learn more important lessons than how to break a slump.
When choosing the spot where you do offer help, don't pile on. The resources and suggestions you list all look great but if you throw the kitchen sink at him, it is more likely that nothing will stick. When we get a hitter with mechanical issues, we can only concentrate on one, maybe two things at a time even though we see five areas that need fixed.
I believe your son is a junior, if I recall correctly. Don't buy into the short-term "poor me" panic mode that teens can go thru. Dismiss it by reminding him of the good player he is - that he is still the same player that earned the starting spot in the first place. Remind him of why he likes playing baseball. I know it can be tough as a caring parent but don't fall down the hole with him. Show that you have confidence in his abilities and he should too. Keep it short and let him get to work.
He should work to earn his spot back by outworking everyone at practice and focusing on the specific task at hand for every defensive rep, every cage cut and on-field AB. If he earns a spot back in the line-up and it is lower in the order, he should appreciate that he'll get more fastballs that he doesn't have to take. If he doesn't win a spot back quickly, appreciate that the team has enough good players to fill his spot until he does. Always positive. Always working hard. Always focused on the moment. This is how good results will come. Even if they are a different kind than what the original goal is.
Don't let him (or yourself) fall back on "yeah but he has a great OBP", "the team's psyche was disrupted because coach started freshmen over seniors", etc. HE needs to figure out specifically what he needs to do to earn his spot back and work hard at it. Meanwhile, he needs to be a great teammate.
Best wishes..