Next time you walk past be ready and as he reaches out to nibble give him a tap on the nose and a harsh no. If he persists increase how hard you tap his nose. He should get the message.
My horse Savannah went to bite me in frustration for not having a treat. I learnt not to hand feed treats but put them in a bucket then allow the horse to have them. Back to biting, as she lunged out I gave her a bunch of fives right on the nose hard enough for her to jerk back and look at me.
She took a couple of days to bring her head down again but she never tried to bite again. I just treated her the same as the herd would.
Don't read this wrong, I do not hit horses on the face or any where else for that matter and have been known to tell anyone off who raises a hand to my horse but there are times when herd mentality has to be brought into the mix.
Savannah also tried to kick me with both back legs. As luck would have it she missed, I did not. I had a stick in my hand at the time and as she missed I lashed out hitting her on the hind legs. She got the message and never tried me again. Its a respect thing, if she had of got away with the kick I would have been forever in danger, instead I had a horse that was gentle with me. Savannah is the horse at the bottom of the page
The "rescue" mare also tried biting me, twice in the same day, out of fear. Same deal. She hasn't tried again. Her colt has not gotten to that point in any way. He sniffs shoes and halters, but that's it.
I guess my main issue was, "what will the owner say...if I discipline his horse in some way?" That's why I like the brush idea...I'm not contacting him, he's contacting ME, and finding out I have unpleasant, prickly bristles. ;)
Hmm, seems like so far, mares are "smarter" when it comes to this...