He has weekly lessons with one of the pc instructors. After months of trying to sort it it was her that suggested a change of bit. He has got a lesson on friday after school so we are going to have a play around and see if we can find something to help.
Why not get some lessons instead of changing the bit? I am not an instructor but some basic things occur, e.g. doing one jump and stopping right after it, jumping with placing poles to teach both of them to collect and extend the canter, doing the course in trot, etc. I am sure a qualified instructor will have tonnes more good ideas to help without resorting to a stronger bit (which risks teaching your pony to become stronger and your DS to worry about it more).
Watching with interest as DD1's new pony loves to jump and can get very strong over the bigger jumps.The pony is in the same bit and flash as your DS's,but no martingale as he puts his head down and just goes,so a martingale wouldn't help.
Ds (7) is having problems controlling his pony jumping now they are doing bigger tracks (2ft plus) she just takes hold of the bit and goes. They had team jumping practice today and she was really nappy and strong the jumps that he had her were lovely and he did get her round the course in the end! She is in a Myler hanging cheek at the moment and has a flash with running martingale. She has a very soft mouth most of the time he just needs a little bit more jumping.