My horse Delta has long hairs around her muzzle, which I never ever trim.
They act for her as very, very sensitive feelers. She can't use her feet to touch, so she is using her mouth hairs.
Horses can see - but not quite as we humans see.
They can hear and have better hearing than we do.
They can smell and have a higher acuity than humans - especially for pheromones which are very faint odours which we as humans give off unwittingly in times of stress or emotion
They twitch their ears and use facial expression as a form of body language.
They posture as a form of expression
They can taste very subtle differences.
They lick things including humans especially for salt.
They feel vibrations in the earth especially if the hooves are steel shod
They can recognise humans from a distance by the way they walk
They can sense almost all of your body movements whilst you are in the saddle.
They can sense, fear, tension and excitement in their rider.
They can judge merely from the pitch of your voice some of your meaning.
In other words horse can receive communication from you and not necessarily by way of speech.
However the big conundrum is to know what exactly they are trying to communicate back to you, their owner and rider.
If my mare were to put her eyes that close to me, then I would take it as a compliment because she is making herself very vulnerable to me.
Equally I would try to guess at what she is trying to communicate. If she wants the pleasure of touching me- then she can touch away.
But personally I'd want to go further. I'd definitely try stroking her, breathing on her nose, talking to her in whispers. I'd also have a carrot ready to give her to see if that is what she is asking for.
As it is, all I get from my mare is a muzzle nudge which means - and I can say this for certain - "where's my horse biscuit!"
I cuddle her neck, I stroke her constantly, I breathe up her nose, I stroke her mane, I tickle the hairs around her muzzle. Often I put my forehead to her forehead. I get as close as I can - but often to the dismay of some horsey folks who say: "don't enter her space or allow her to enter your space".
Horses are ultra sensitive in many respects -a light touch often conveys as much as a heavy touch. A soft voice as much as a loud voice. A subtle aid/cue is mostly all that is necessary if the horse is listening to you.
Does he do it with anyone else?
Is this communication to you personally?
Until I had reason to think otherwise, I'd think your horse was being nice to you. Don't knock it. Just watch out for his steel shod feet.
PS It might even be love and affection!