Scratching his itches which he enjoys is not mutual grooming which I was thinking of when you said it. Mutual grooming which they do with each other is the big no no with you.
If he loves it this much you will probably find that brushing him over daily with permoxin will take away the itch. We do that after every ride. I don’t know if you have that over there.
Every horse is different. Every horse is unique. Normal exercises do not work on them UNTIL,
they start getting over the basics and start understanding and then suddenly the exercise that did not work before works now. This is why something that you tried before, that did not work may work now.
Also don’t forget that every horse has read their own book. Not our textbooks. The horse book they share with each other. This is not the book you want them to read.
That is the book you need to understand and it is not written in English, you are told about it with every horse you interact with. This is why good trainers can go from one horse to another. Exactly how a good musician can go from one instrument to another with instruction.
You can’t tire out an adrenalized tb. So you never try but it should not happen again and if it does with side reins he is working himself and less likely to injure himself.
So what would I do?
Grassy hay only. 24/7 turn out. Lunging twice a day in solid side reins at the GOD. Controversial all of that but elastic side reins can teach them to pull and lean. Twice a day as he sounds fit and strong.
Lunging without side reins causes a very fit horse but not a horse that is in work. After months the rider gets into trouble as the horse especially an otttb gets too fit for the rider. Not only does lunging without side reins cause this, but so does a hot horse that is ridden and not worked. Eg walk, trot, canter and jumping jumps only warms them up and is not work. That is also where a lot of riders get into trouble with ottbs after months of riding. I am talking generally not saying yours is a tb. I am also not saying all tbs.
You do not sound like you have this problem with your riding. You sound like you can work a horse. I have not seen you ride.
I would lunge twice a day at the GOD, just like you did, until there is no reaction. No reaction to GOD. No reaction to next door neighbor mowing. No reaction to horses galloping in the field. No reaction to gale force wind through long grass. No reaction to tripping, in my case with my mare, over hard cow manure, slipping on wet cow manure or if a calf came to look at her. If a pelican flew overhead. If she went in a dip on the ground. The list goes on.
I started lunging her as hubby bought her without me for him and saw me on her one day leaping from side to side in canter and banned me from riding her alone. Which of course I am 5 days a week.
So I lunged her in the solid side reins. I never pull them in. She was trained western, she went with her face horizontal to the sky, there was 3 feet between her back hoof steps and her front ones and she bucked at EVERYTHING. I mean everything. She bucked at the saddle so I changed it to a roller. She bucked for 3 days. I put the bridle on. 3 days. I put a loose side rein on. 3 days. You get the picture.
I lunged her just as something to do after I had ridden the gelding. I stuffed jeans with newspaper and tied it to the saddle. I put boots in the stirrups where they wouldn’t hit her elbows and let them dance around her. She had been ridden in a stock saddle with their feet on her shoulders and she was reacting to my legs on her side. I figured this out one day when she reacted to the rein touching her neck. I stayed in halt and touched her all over to show her that being touched did not matter.
I only lunged her once a day. We did not get to no reaction in 3 days. Sigh.
Then one day I realised that she had not bucked for absolutely ages. I was now looking at a dressage horse. She WAS TRACKING UP. I actually cannot tell you how long this was but I started lunging her down in the cow paddock and by now we had built a dressage arena. Lets say months but yours is a lot more advanced than she was.
I hopped on her. Nobody else around but she had told me that it was okay now. She was wonderful. You really know when a mare has bonded with you.
She never bucked again. Not the day she stumbled, went down, came up and just trotted forward.
Not the day that she slowed coming to the centre line and I thought it was because the sand was a bit deeper and non verbally assured her that everything was okay and to trot on. Along the rail I glanced down and I now know what they mean when they say that your life flashes before your eyes.
It really happens. I had seen that the side rein, that after lunging I had taken off the bit and attached it to the d ring at the top of the saddle, which incidentally I had been taught to do. Had detached from the d ring and was now swinging between her legs as she trotted.
I don’t actually remember doing it or how, as I said my life flashed before my eyes, but I asked for halt, bent down, picked up the side rein, reattached it to the d ring and asked for a halt trot transition and she trotted on as if nothing had happened. As I said she never bucked again.