After trying a variety ...in a thick-tongued highland...we have settled on a hanging cheek snaffle with a lozenge.
And believe me, I now have enough bits to start my own bit-bank.
This seems to be comfortable for him, and I was told it is becasue it is more suspended in the mouth and a bit better for those with not much room. He now has a lovely foamy mouth when schooled and goes really well with no fidgeting or head tossing.