I like using a Skito Correction pad
with a long shoulder shim to help fill in for a horse that is lacking in topline. This pad cushions the saddle well and fills in for that lack of muscle. The horse is then more comfortable, which allows him/her to use his back and lift his belly, which will help build proper topline. As the horse fills in, you can remove or replace the shims.
To help build topline, work during warm up and cool down in a long and low frame. Keep the horse "marching" forward, so you feel the back come up under your seat. Ask for the horse to hold it a few strides, then relax a few strides, back to holding long/low/forward a few more strides. I will do this at the walk and trot only at first, moving to canter work as the horse improves and can hold the farm longer. Just remember to bring the horse's head up into a normal position between "stretches", so he doesn't think you want him there all the time.
Belly lifts can be helpful before and after your ride. Google "horse belly lift" for instructions. You should be able to find a video demo on youtube.com.
Lunging the horse in side reins set low will help build topline. I can give you instructions if you've never tried it. You use thin split reins tied to your cinch rings if you don't have side reins.