It is very difficult to advise you as to what will fit his shape without actually being there .
I would still like to see the wintec dressage saddle that is at the yard on him.
A few basic rules for you to apply when fitting a saddle
A saddle must conform to the shape of the horses back - the panels must be in even contact over the full length of the saddle and the front arch must be clear of his withers without pinching . If you look from the saddle nail downwards you will see a pocket that contains the point of the tree , from the nail to the pocket is the part of the tree that must be the same shape as the horse at the front.
Any saddle must not restrict the potential for the natural movement of the horse - it must not interfere with the movement of the shoulder
The gullet of the saddle must be wide enough so as not to come into contact with the horses spine.
If you can get a hold of SADDLERY by Elwyn Hartley Edwards, which has a good section on saddles and saddle fitting, and if used as a reference will help you in choosing the correct saddle shape yourself and Nelson.
When trying a new saddle as before - lunge the horse on both reins , walk trot and canter and make sure you can see no movement of the saddle on his back , and look to see an even mark left by the saddle after work.
The saddle should look right on Nelson - it should not appear to be tipping forward or back , the deepest part of the saddle seat should be in the middle.
LISTEN to Nelson - he will soon tell you if he is unhappy in his back .
Sorry I can't be there to help you , but if you have any pics of different saddles on him I will do my best to help.
And good luck