Dry spots by the withers are caused by a saddle that's too small. Adding a half pad will make the problem WORSE. I would suggest using the thinnest pad you can find and get a ThinLine Contour Pad
over it. The TL pad will help ease the pressure on his withers while you save up and look for a new saddle.
You can't fix a tight saddle with a pad, at least not long term. You just need a new saddle. Or ride him in a wider one while he loses weight and gets back in shape.
Once you find a saddle that fits, or even is a tad wide, I like THIS ONE
, or if you're on a budget, THIS ONE