a saddle pad might not fix that. try a saddle fit
check saddle fit. only use a riser pad if the back of the saddle is lower than the front
If it is his shoulders that are pushing it back then it probably won't stop it from slipping back. It might just end up tipping the the saddle forward and down onto his withers. You might just need to get a saddle fitter out and get it slightly refitted. He could have changed shape since the last time you got it checked.
You can also just get the saddle more padded in the back so that is is higher and doesn’t slip
Maybe trying a non slip pad?
Add on. saddle seems to fit fine other then the sliding back. I notice his massive shoulders push it back. if there's any other advice it would be nice. I will buy another saddle if this cannot be sorted but would prefer not to. cheers