this is a really easy fix.
first what kind of mounting block?
does it make noises?
I have rebabilitated in ny alone 20 otb who never kneew how to stand still.
I feed them off the block,
I even put block in round pen and lunge off the block,
every time you walk your horse and he stops even for an instant-give one treat.
the last thing u want is to push him and make it worse.
it takes time and pptience.
do the stops with out tack for weeks until he stands for 3 mins ,
then with tack,
u must get him used to u standing on the block,l
even move it to his staoll and stand on it while he eats,
make the block his friend.
then when all anxiety is gone try with a competant handler on the grouon and off
on and off.
just do not push too much to fast.