Mare, gelding, stallion, weanling, yearling, foal..... yes folks it's that time if year...... check for lice. Make sure he is dewormed.
Next guess.... could bee a pain response from nick, facial, back, knee, leg, hoof issue, Navicular, arthritis, calcium deposit, some type of nerve issue that's bugging him where itching will temporarily relieve his pain.
In a nut she'll rather than home remedy and trying to diagnose from strangers over the internet, have your vet check! Your farrier can rule out several issues but now that springs turning into summer and the weathers getting warmer the vermin are hatching from last year and getting into digs, cats, sheep, goats, horses, cattle etc. vets have been getting busier and busier as the season warms.