Saddle fitting...help!

Apollo has no shoulder muscle what so ever so my saddle falls down on his shoulders and puts pressure there. There is a 2 finger clearance on his withers but he has no muscle to hold up the saddle so the saddle is at an angle...(I have not ridden him in it...just put it on his back with no pad and I could feel the slight pressure). I'm pretty confident it will fit him once he gains his much needed muscle, but in the meantime I don't have a saddle to ride him in that fits him properly because of his lack of muscle.

I was talking to a couple of people at my barn, and had them look at him with the saddle on, and they suggested this pad. (which I just ordered)
Wintec Comfort Saddle Pad With Lift - Statelinetack.com

Is there anyway I can help my saddle fit better? Do you think this pad will help?

I just want to see what I can do (if anything) with the saddle I have before I go out and spend money on another saddle that probably wont fit him once he gains the weight/muscle he needs, since his body is going to change so drastically...

I will try to take some pictures tomorrow to give you a better idea.

It would be helpful if you could post a photo of your horse with the saddle on. Adjusting saddle fit with pads can cause pressure in different areas. Lifting the front of the saddle off your horse's shoulders can shift the saddle causing it to put pressure in the back and to put in the wrong position when you're riding. So, when you put the saddle on with the pad, check that it sits so that the balance is correct. Here's a link to a page with a 9 point check list for saddle fit.

Another option is a saddle pad with pockets for shims. These pads allow you to use shims to build up the front or back of the pad without causing imbalance in the saddle fit. This page shows the Thinline and Mattes brand pads.