Pearlsacarolsinger
Up in the clouds
Did you not check the saddle for fit before your son first rode. Surely if its not the one you tried him in alarm bells would ring as ponies rarely have two fitting saddles. Personally I would lunge him in an long outline gently building up some topline no saddle or son. Then get a saddler to either find a saddle and make adjustments or make one from scratch. But if he has no topline don't do it straight away because is shape will change with proper work. We've had a couple with bad backs and just what we have done but I would say avoid pain relief where possible or ppnies will continue to work in a way that will hurt them once there is no relief so build up the wrong muscles. This also happened to me when I hurt my back. Really bugs me how people do to ponies backs what they wouldnt dream of doing to horses.
I hadn't picked up on the fact that you hadn't got the tack you paid for. I'd go back to the vendor and ask for the tack that you DID pay for. In fact I wouldn't have accepted the dodgy tack in the first place but it's too late for that now. I'd also be telling the vendor that their actions have caused the pony to have a bad back and you'd like a contribution to the vet bill. I don't expect you'll get that but it might mean that you get the correct tack. Failing that I'd at least want the £450 back.