I feel like I can answer this as my son received his Eagle rank last year...
ANY kind of gift would be nice...these young men have worked many years to receive this award. So many just come to the ceremony and don't do anything to honor these Eagle Scouts.
Of course money is great. Just because he's an Eagle doesn't mean he won't still be a Scout and money (if put to a worthy cause) can be great. Our son put his nearly $500 towards attending National Jamboree this month. This trip alone was $1300.
Gift cards or anything camping is also a winning idea, I have to respectfully disagree with the above post, you can never have all the "supplies"/camping gear you need. We've spend over $250 in supplies for our son's 2 week camping trip to National Jamboree and he's been a scout for 10 years. Not to mention camping gear wears out too!
You can also buy anything with an Eagle on it. I've gone to my Hallmark store and gotten wonderful books, picture frames, sculptures and more .
BSA also has great Eagle Scout recogntion items, scrapbooks, baseball caps, and other items.
Lastly, and my and my favorite. 
Call Buck Knife..1-800-326-2825
Ask to speak to the person who handles the ordering of Buck Knife Eagle Scout knives (it is a lady or was last year). You can order a beautiful buck knife with the scouts name and the Eagle Scout logo on it for around $30.00. Turn around is very fast.
Of course with any of these suggestions, I'd check to ensure the familiy isn't arlready getting there Eagle the same gift.
Good luck