A riser pad will only make it pinch at the shoulders worse. I would recommend using a ThinLine Half pad
or Contour pad
and moving the saddle back about an inch. That will help give his shoulders more room to move and the ThinLine pad will eliminate any pressure points.
Vets don't know anything about saddle fit
. If you're really worried, have an equine chiropractor out (one that's also a vet) or a professional saddle fitter.
If you post some pictures, we might be able to give you an eyeball opinion. Put the saddle on, no pad, lightly girthed up. Pay attention to where you're placing it. The front d-rings should be 2-3" behind the back edge of the horse's shoulder blades. Take a side shot showing the whole horse, stand on a stool and tack a rear shot, and take a 3/4 angle showing the WHOLE SHOULDER, so we can see how the saddle lays against the horse's shoulder. Make sure the horse is standing on level ground. With his head up and straight ahead.