Good place to learn and you just might be suckered into the hobby yourself!
Metal detecting is a very beginner-friendly hobby because there is a detector for every budget! Like anything, you get what you pay for in terms of quality, but there is no reason a cheap detector can't find plenty of "goodies" to keep your grandson interested. If he hasn't detected before and you're hesitant to make a big investment, metal detectors can be purchased for as little as $50-75. One that comes to mind immediately is the Bounty Hunter Junior detector which I know many have started their kids on:
https://www.amazon.com/dp/B01J9M04BO/ref=sspa_dk_detail_0?psc=1 If you find the interface to that one is a bit childish-looking, the Bounty Hunter Tracker IV is around $90 on Amazon and I believe Harbor Freight may carry it cheaper:
https://www.amazon.com/Bounty-Hunte...4062&sr=1-1&keywords=bounty+hunter+tracker+iv Can't vouch for either of these machines myself, as I've never used them, but the reviews are good.
If this is a hobby you feel you'd like to learn alongside your grandson or you have the budget for it, I personally vouch for any of the detectors in the Garrett Ace series, which I myself started with.
https://www.amazon.com/Garrett-Ace-...e=UTF8&qid=1512511320&sr=1-4&keywords=ace+150 These range from $150-350, less from the right dealer.
I see you're from Pennsylvania... if the weather there is anything like it is right now in Wisconsin you may have to wait awhile to get much detecting done as the ground is already starting to freeze (frozen from an inch down here), although of course you can always practice with metal objects on top of the ground.
I would like to add how much I admire you for wanting to help your grandson get involved in a hobby like metal detecting. Not enough kids today have hobbies that are healthy and get them out into the great outdoors. I remember one of my favorite activities at your grandson's age was carving sculptures with my pocketknife and going on nature hikes. Not enough hobbies like that today.
Glad to have you on the forum. Hope my post was helpful. :-)