S. Young, I love this topic. As a parent, I always feel helpless when he struggles. He's 16 now so much more mature and able to cope with a slump. I do remember a couple of things that helped him:
1) He had a terrific travel coach when he was aged 10-13 who told him something I've heard many times since: Baseball is a game of failure. If you succeed 1 in 3 times you're doing very well. His coach also told him that he didn't have to carry the team on his shoulders every game.
2) The only thing I ever said that made him feel better was that he only spent a small percentage of the time in a slump and if he's doing pretty well 85-90% of the time, then he needs to take that feeling of doing well into the next at-bat and not give in to the fear. But he had to experience a few slumps before he could logically understand that.
I would love to hear other people's perspectives on this too, especially as their son grew older.