I would suggest consulting with a saddle fitter (of some form) as well – I have a TB who sounds similar who was also tough to fit. I’ve had good luck with a Courbette flex tree saddle on her – others with spring trees fit (often with a half pad with thinline shim in the back), but she is the most free and comfortable with the flex tree. If he changes size often, I would also consider using a shimmable half pad system like the thinline trifecta (LOVE mine!) so you can help prevent soreness as he changes shape. Though my saddles fit both my mares well, I like having the thinline under the saddle for both my own comfort and peace of mind for them.
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