Just a thought but has he been going through this stage from he has had to wear a muzzle?? It could be frustration, how much turnout is he getting...I know you spoil him rotten and love him to death but I am just thinking that I remember seeing the pics of him wearing the muzzle and then all of a sudden he starts playing up again??
He wore the muzzle all last year and all the year before. The first year, he wore it from July to October and it ripped his face to bits (as I was still in the stages of trying to make it work for him where it didn't rub) but he didn't act up like this BUT he wasn't in as much work. The year after (2016), he wore it from May right through to the end of July - ish time and he very occasionally snatched whilst out hacking but one mildly and never ever in the school or jumping paddock and never ever while being lunged or anything. This year it started in the second week of May-ish and has progressively got worse and worse. It's not getting worse now but it's not getting much better. It's not his muzzle, I'm 99.9% sure of that otherwise he'd have done it the whole time and he hasn't.
He doesn't ever do it when I'm doing Parelli/Natural Horsemanship with him, he's good as gold if a little sticky footed when I'm doing back up and stuff but that's fine. He never does it leading to the field or coming in. He doesn't do it when he's being washed, groomed (even when I brush his face and such) or messed with. It's only when he is asked to work.
So I tack him up or put his Equiami on and he's fine. I lead him to the school/round pen and he's fine for a minute and then he starts to do the tiniest of snatches which I ignore. When I get into the arena/pen and stand sorting out bits and bobs i.e stirrups, girth or the ropes on the equiami, he stands like a lamb and doesn't object, no snatching at all. He stands by the block like a lamb and when I mount he stands perfectly still until I ask him to move forward. I can hop on and off as many times as I like and he will always politely stand still until I'm ready to go. When I walk off, he's fine for the first minuted but then as soon as ask him to walk more forward or do anything requiring him to use himself properly, he starts to snatch and it gets worse and worse and worse until he is snatching and stamping his feet, overbending himself behind the bit, chomping etc.
He also does this grunt/snort - I call it his dominance sound as it's exactly the same sound he does when he tells off the other horses in the field, I've watched him a million times. He also does it when he's trying to stamp his authority with me in the stable, particularly at feeding time (but it gets him nowhere because I get in his face and move his feet and he knows better than to argue with me as it's not worth the hassle for him) because he is, and always has been a very, very dominant pony. This is why I think it could be behavioural because his snatching and grunt mimics the dominant head shake and grunt he does at horses and occasionally at me.
I have ridden him with a contact, with no contact and as a last resort, on the buckle and it makes no difference. I have checked and rechecked his bridle fit and got several opinions off m instructor and saddle fitter and others and it's not too tight and fits him well. He is healthy and not overweight, has excellent feet according to the farrier, he has never been lame in the 3 years I have owned him, he's never lost a shoe, he's never had any serious back issues, just a bit of soreness in his neck once and a bit in his back earlier this year which I had sorted out. Physio gave him a thumbs up, saddle fitter did his saddle two weeks ago, teeth are up to date, everything is how it should be, I meticulously take care in his routine and suchlike, I spend way longer at the yard than others because I go to such lengths to make sure all is how it should be before I leave. I do not have a clue what else I can do.
I'm about to buy him a nose net to fit his grackle bridle off ebay as there is a girl who makes them and I'm going to try ride him in a fly mask as an experiment to see if that helps. I've also booked in a huge session of lessons and if that doesn't work and I reach the end of the block, I'm going to call the vet out to get him checked and if all is clear, considering putting him on a temporary calmer to see if that helps.
Also what makes me think it's behaviour is:
a.) It only started in May and at the same time that he started galloping flat out across the paddock when I turned him out - spring grass?
b.) It's worse with other horses about - like it's made worse when stuff is going on
c. ) He has ok days where it's not so much and really bad days where it's terrible
d. ) His eyes aren't weepy, he's not snorting or sneezing
e. ) He's happy to go for a walk in hand in the school/up the lane etc as long as it's not work
My head is battered with all this lol