There’s something called “state dependent learning”. If he was drunk when he hid it he might well remember where it was if he was drunk again. Whether the risk of drinking again is worth it he’d have to evaluate. I’d stay sober and keep looking.I am 63 and have lost things my whole life like that. It never gets better.
Friend of mine hid 4000.00 2 or 3 years ago , still missing. Back when he drank a lot. He quit drinking right after he hid it , so ??