Fitting a saddle to a Quater horse

hey

I have recently purchased a paint horse, he has fairly big shoulders and is quite solid but not massive. I am competing in dressage and showjumong so need a dressage saddle on him but the one I have at te moment slipps around a lot!, side to side and BACK! it seems to be sitting toooo far back although I do realise it has to sit behind his shoulders.

The thing is that white hair is growing through with hard lumps so obviosly is is NOT fitting him. I have stopped using it but need a new one ASAP!!

I dont know what to do, it has a changable gullet but doesnt seem to make too much difference.

Does anyone know of anything that will help or brands/types of saddles to try>>

pleasee!!

thanks

 more pics as well