Does he bite when you are on ground with him, e.g. leading, brushing, touching? If it’s not pain related I’d recommend, free lunging or doing groundwork, lots of it, most likely he lacks respect towards you, or is telling you to move. Praise him when he’s being good, lots of it, and be nasty when he’s nasty towards you, if a horse ever came viciously at me with teeth or tried to nip me it’d get my elbow in its face as an act of self defence. If you’ve never freelunged/ done groundwork I’d recommend watching some videos on YouTube or asking someone on the yard to help you :)
The horse I work with is quite the sourpus. He’s grumpy, which is one thing, but can be aggressive which is another. He doesn’t seem to be uncomfortable or in pain, and goes just fine under saddle, but constantly tries to nip and bite.
I’ve tried smacking him when he does it but it doesn’t help and I’m worried it just escalated the situation which I don’t want to do.