He may grow out of it. My gelding was horrifically mouthy until age 6, he never bit or nipped anyone but he just couldn't keep his mouth off things (and he got a ton of hand-fed treats from my kids).
He finally got the hint except when you pick out his right front foot or the farrier is working on it, he snuffles under your shirt and LICKS you! If you tuck your shirt in, he will attempt to snuffle it back out and if he can't, he will spend the entire time snuffling around on your shirt with his lips. Stand like a rock for the other 3 feet.... it's just the right front and he does it to everyone... I have given up.... he's happy, content, doesn't use his teeth and it's only that foot. I just have to remember to warn other people because it's a bit freaky when a giant horse muzzle start snuffling around your back/butt!