Dooer...
Lot's of good advice here. I've seen similar occurances when stud underclassmen become the senior leadership on the team, and feel the need to carry the team. They start pressing, and trying to do more. They stop allowing the game to come to them. Then a slow start to the season increases the anxiety and makes matters worse.
Ya gotta relax.
Easy to say... Not so easy to accomplish when, in your mind, so much is riding on your success. I think you've gotta get the focus off of your own success/failure, and redirect to the success of the team.
I'm not suggesting for a moment that your son is not a team player, or that he is selfishly placing his own goals above those of his team. On the contrary... Players can be so driven to help the team that they agonize over any element of their game that falls short of their own expectations. Always keep in mind that there are eight other batters in the lineup. ...eight other defenders on the field. They've got your back, and know that you have theirs. Relax and trust in the team. And when you have a bad stretch, be it one at-bat, or a long string of them, stay focused on the team goals. Play within yourself.
Sometimes a different spot in the batting order can get you different pitches, or simply alter your perspective. Whatever... ...different answers/solutions for different players. Talent will prevail.
Case in point...
Senior on my son's D-1 team... Flat tore up the league his freshman and sophomore years. Followed up with a very dissapointing year last spring in his junior year...approaching the draft...upper classman...(insert reason of choice here). Who knows. ...and ultimately, who cares other than him.
He opened this spring with a 1 for 23 stretch that would be devastating to anyone, let alone a proven team leader. Got benched, but the coach never gave up on him. Kept getting him back in the box as much as possible. He finally had a breakout game four weeks ago, and has never looked back. His turnaround has been remarkable since that time. He's been on a tear, raising his average from a dismal .043 to well above .350. Now has the second highest BA on the team. In fact, he's leading the team, (and perhaps the SEC) in conference play, with a blistering .630 average against conference oponents.
When pressed for the source of his turnaround, he said something to the effect that he had to get to the point where it stopped bothering him. He stopped dwelling on past failures or the last at-bat, turning his full attention to a right approach for the next at-bat only. Nothing else mattered.
Baseball's a funny game that way. Can't play the game the right way if you're too high or too low. You have to treat good days and bad days alike. Leave 'em in the locker room when you go out to the field. ...cause the only important at bat, is the next one.
Dooer... Your son will come around. Don't know if it's time away from the field, more time in the cages, lighter attitude about the game, hypnotizing, breathing through your eyelids, or a well-timed distraction as noted in another post above. Different strokes...
He'll be okay when he let's go of the minutia that you can't afford to bring to the plate with you. All the best to him and his team for an outstanding second half of the season.
Now, as for his parents... BEEN THERE...GOT THE TEE SHIRT... I recommend sedatives until the ship is righted, and all the positive reinforcement you can muster. It will not be easy for your son to set aside his anxiety if you are pulling out your hair over it.