It' usually means pain somewhere, could you try a thinner bit or it could be a habit he's gotten into. could you try a hackamore on him and see if he grinds his teeth with that.
This is a young entire horse, just in for breaking he has always done this as was shown at 1 & 2 years, he is a very sweet person and is happy to to all that is asked of him with no argument, just the grinding, we have tried happy mouth, thin bits, thick bits and are now trying an angled bit.
My old pony used to grind her teeth, always did, just seemed like a bad habit, the older she got the less she did it, but I am talking in her 20's!! It never caused her any harm, she would do it whenever she felt, not necessarily when she was tacked up!
my mare does this when in any bit bar a myler combination, no idea why, even now when i put her in her KK snaffle for dressage she does it still.
apart from dressage i now always ride in the myler comb, she did this when i bought her and i did all the normal checks teeth/back/tack etc and there was nothin wrong so i then started trying difo bits.
she is a tense horse at th best of times and when stressed dose it worse, but never ever in the myler,I wish i could ride in it all the time but of course its not dressage legal.
when does he grind his teeth? TBH I think it is a stress/discomfort issue. Who has checked his teeth and has he had regular dental treatment? If stress then he may not appear stressed/in pain but could have been in the past and is now just a force of habit such as box walking/crib biting etc. I don't think there is much you can do to stop it tbh. Does he do it int the box only? I knew of a mare who used to do it when stabled but not when in the field so is now happy and turned out 24/7.
If he will not stop then please get a qualified equine dental technician to check them regularly as they will wear away/cause dental problems.
He was ridden today on his own for the first time, and was fine no grinding, however when still even if not on his back he will grind, it would appear that if he has something else to think about he forgets to grind.