What's the best way to help a man through a midlife crisis?

Let him work it out on his own. There's nothing a mother can do that will be effective. If he wants to buy a motorcycle, or go on a long vacation cross country, that's all to the good. He will work it out of his system.

If, on the other hand, he is dumping his wife and kids for a 19 year old blonde, then you will have to have a mother/son talk...of the "this, too shall pass" sort.