The way it sits, he definitely needs a riser pad to help level out the saddle. If you think it is too wide for him, maybe try a memory foam type half pad. One that will conform to his body and fill in the space where needed. Also, if you dont use one already get a breast plate. I like the ones with the elastic but if you show hunters you will need a traditional leather chest plate. My gelding has the same problem his withers are high, so i ride him with a chest plate, baby pad and an ogilvy pad(it was a gift) or a thick fleece half pad. My saddle generally stays where i need it to. I have an m toulouse and it fits well a little wide for him but the half pad makes up for that. If buying a new saddle is not really in your budget i would look into a good half pad. Maybe ask around your barn or other horse friends to try different half pads to see what helps then purchase the best one. Good luck!