I noticed a bunch of white hair below my horses withers on his back, I heard this is from saddle pinching, what should I do ?

If you think your saddle doesn't fit you should find someone who knows how to fit them and get a new one!
Your horse might not tell you outright that it does not fit. He might just not perform the way he could.
Usually white hair is from a saddle that does not fit correctly. It could also be from a not-qujte-clean spot being rubbed then growing back white. It is more likely from an improperly fitting saddle though
Does your horse show any signs of being pinched while ridden? (Pinned ears, reluctance to go forward, bucking, or maybe just simply stiff). If so, I recommend a lifter pad. It may be wise to consult with a saddle fitter as well to see what can be done.
Is it possible they were old scars. If not you can follow Cassidy's suggestion, but the best thing for your horse would be to buy/borrow/find a saddles that better fits.
If you don't want to spend a bunch of money on a new saddle for your horse another thing you can do is buy a saddle pad that has better padding in the places your horse needs it.
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