Well, good news. The 2 of us went over to one of those "mall" sporting goods stores-one that happens to carry Giro helmets. Turns out that some Giro helmets (those that come in "L/XL" size) just barely fit him (the "universal fit" models don't). The store wanted something like $69 for a havoc, which was steep, but at least he knows that he can order any of 3 or 4 models through mail order now.
Still, I'm surprised that there seem to be zero helmets available in any size between these and that big Bell bucket. If there is enough market for something that big, then ther is certainly a market for something in the in-between sizes.
FWIW, though, I'd bet that most of those king-sized Bell helmets are bought for women that have "big hair" and need something to fit over it.