dot to dot is unreliable, as they aren’t in the same place with every saddle, and can even vary a bit across the same brand. And, as stuffing/flocking compresses, will widen a little.
Prestige comes in cm widths, you need to use that as a starting point, but it’s really only useful when comparing Prestige saddles, or for getting a ballpark idea with other brands.
I don’t know about new Prestige saddles, but older models do not have plastic trees
Your guy looks like he needs his back muscles “fluffed”. It looks like there’s a “ledge” above his withers where he flattens out - is that reality? I don’t see why a Prestige isn’t a good start. His back is flatter, but he does have an upward slant which is where the upswept Prestige panels come on handy.
Prestige has “cathedral” pommels, meaning lots of nice wither clearance. But he also looks to have hollows along his withers, which often goes hand in hand with that “ledge” along his back, either due to long-term incorrect riding, or poor saddle fit, causing muscle atrophy. Shims are very useful for that, and MUCH easier to manipulate over having “trapezius” flocking added (something Black Country does a lot of). You may be looking at a combination of some extra padding, and shims, for a saddle that is the right shape, but currently 1-2 sizes too large, as he develops his back and wither muscling.