Wool socks that aren't the ones with holes in them
He has three pairs. Two of them are finished.
Check his sock drawer. There'll be a rotation of the same few pairs, thinning at the heel, and he'll wear them until they're transparent, because buying socks for himself feels like a waste of money.
Merino wool is the upgrade he'd never make on his own. It holds its shape, it doesn't hold smell the way cotton does, and it stays warm even after his boots have leaked. About $25 a pair. Buy him five pairs and he'll mention it to you six months later, which from him counts as effusive praise.

