He doesn’t need a license, although in some areas that are of historic note, he would probably need somebody’s permission. Virginia, in particular, gets testy with treasure hunters. Here in Alaska, nobody gives a damn where you do it, although commercial gold mining companies would run you off.
White makes a good metal detector. I’m not even remotely competent with the thing, but it works well. Make sure you get one that can discriminate between pop tops and gold, that can tell you approximate depth, etc. The good ones have the ability to adjust to the surrounding rock and minerals.
Anecdote: A guy who works for me bought a new detector and took it out to his worksite. Gets out of the truck, turns it on to adjust it for local minerals, but can’t get the damn thing to quite shrieking like a banshee. Bangs it against the truck tire a few times, but it continues to light off. Figures, ‘what the hell’ and starts kicking at the dirt where the detector is howling and unearths a HUGE gold nugget.