I would not underestimate you by your picture.
I know what you are saying about a horse becoming head shy if whacked on the muzzle or cheek . some of them do, especially if the first times the punishment is half hearted, and ends up escalating. the truly persistant , mouthy horse will just escalate his tries right along with you.
you can try the holding the nail idea, I think already describeed, to address his all out attempts to bite.
but, . . . about holding his halter. I understand that you are trying to 'keep' him from biting, but I think you may need to allow him to make that 'mistake', and then apply the consequences.
let me back up . . . in general, 'holding' horse so that he does not do what you don't want him to is not a good training approach. the only thing he will learn is to lean or push against your pressure. it's like holding the reins; the horse learns to push and lean on the bit. Holding your leg on him? he tunes it out , or bucks. Holding the lead line taut? hrose just startes to resist the pull on the halter.
the idea is to make things HIS responsibility. so, you address his actions, but you never hold him in anticipation of it. Of course, if you must in order to protect the farrier's back, for instance, then by all means . But if every interraction between you two requires you to "hold" him, then he is not learning what you need him to learn.
do you understand?
I would work on having him ALWAYS hold his head off of you. It means that you will have to stop cuddling his head as you are doin in the photo/avatar.
you stand by him and you do have your hand somewhat ready but you are not 'holding or non-stop pushing on his halter.When he moves his head past the centerline of his body, (to start with),you address that. correct it by putting your hand up and block him. you can tap on his cheek, enough for him to move his own face back into "HIS" space. put your hand down
you keep insisting that he keep his head there, ALL THE TIME.
whether you are grooming, cinching, scratching his wither, whatever . . you insist he keep his head over. And, you move him over long before he has reached around to sniff or bite you.
If he does actually get around fast and bite you, then I suppose a very large reaction of a swift hard smack with a lot of noise and commotion is one approach
this might be a place to start. I venture to guess he is not mean, but curious and playful. Still, it is a bad habit that cannot be tolerated. at some point, he has to grow up and stop with the baby mouthy behavior. you'll have to be excrutiatingly consistent to help with this.