You can always flick them on the nose and "bite back". Or if there is a reason to why he's biting, like the saddle is illfitted, or the girth is too tight. Something could be bothering the horse. Maybe is has stomach pains. There are many reasons they could bite. And before training them out of it (because it is a way of communication for them) check all your equipment and if everything is ok in that area then call a vet to see if your horse has an internal pain somewhere, like the stomach.
Usually if they are girth it is a combination of having one of those neoprene rubber girths that pull on their hair, similar to a rubber band in your hair instead of a hair band. As well as tightening the girth to fast. Do it super slow now that your horse has learned the bad habit and get them a nice thick fuzzy wool girth to start. Every time you go to tighten the girth if they do nothing praise them, if they go in for a bite have you elbox sticking out so they end up whacking themselves on it and keep going. It will take some time to fully go away but it can.
I think it generally comes down to just being firm! When I first started riding I never punished my horse for biting, kicking, but as I did we clicked alot better and she knew whos boss, shes also girthy but im not sure you can really help that I just make an arrrghhh sound when she tries nipping. Just dont get overly firm (abusive) ;-) no bite should be acceptable you never no when a young child will approach them.