more background:
I nannied for this kid when he was fresh from the oven for a number of months.

I also lived with him and his parents when he was 6-10. (He's now 15.) This is a kid I've spent a tremendous amount of time with and who I'm very emotionally attached to.

It's a big deal if you're a Boy Scout. If I had ever become an Eagle Scout, I wouldn't have thought it was weird if someone gave me a gift then, but I wouldn't have expected any either.

DO IT.

heh.
No, seriously.
He's an Eagle Scout, he's not suppose to spend it on blow.

DO IT.

Oh, give him a hundred bucks! :)

Do you have to give him money? Can you do something instead that could involve the whole family? $100 could pay for a day out somewhere for a few people and it'd be a nice gesture to do something inclusive.

Yes, it is appropriate. $100 is a bit more than normal, but then again you are very close to this kid, no?

Other things that would be cool—

A really nice compass.
A really nice sleeping bag.
A decent one-person tent.
A copy of the original scouting guide (it's full of crazy anachronistic stuff, but also good tips).
Any of the Patterson wildlife guides.
Big forestry service or geological survey maps.
Quick-dry clothes.
A really nice utility knife.
An electric firestarter.

Or yeah, just the cash.

When I became an Eagle Scout, two of the local banks gave me a $50 savings bond. I wonder if those have matured yet.

The most meaningful thing I was given was a binder from my father. It was letters of congratulations from my congressmen, my mayor, my principal, athletes, astronauts, movie stars, the president of the United States (George, Sr.), and dozens of family friends and relatives. Reading it, it was the first time I realized the importance of the honor.

Perhaps a personal letter of awe and admiration for his accomplishment is a suitable gift.

BTW, I have a current list of all the addresses and contacts if his troop didn't request these types of letters for him. I'd be happy to send it to you if you'd like. Let me know.

The letter from the President is cool, but the letter from the Pope is super groovy.

I don't remember any gifts at my cousin's ceremony, but they wouldn't have been out of place either, so I would say feel free to give him any gift you like. Actually, if you search for "eagle scout gifts" it turns up a whole bunch of eagle-themed plaques, plates and even an eagle bolo tie, so maybe gift-giving for this occasion is common enough.