Sliding back could be a couple things. Yes, it could be too wide for him. But there could also be other things that are causing it to slide back. Would almost need to see it in person to tell.
Although you could take some good pictures with just the saddle sitting on his back (no pad) so we could see how the saddle itself fits. If a saddle fits well (like it is supposed to) you really only need a thin pad under it.
When he does get a good workout, is the sweat pattern even on his back?
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.