As far as metal detectors I have no idea and no clue about them.
I may have him a side hustle with it though. Like just last week my mother text me and wanted to know if I had one or knew anybody that did. She had lost her car key she believed between the car and the front porch. Yeah in the grass. Her and my brother looked for like an hour and then I guess my brother racked around the grass for a bit too with no luck.
She said last time she lost her key that cost her almost $300 for a new one. I lost my truck key once but it wasn't as fancy of a key. Had to have someone come to the house. I think I spent $200 for two keys and to have them programmed.
Maybe he could make some cash finding lost keys or wedding rings and such.