Horses similar to his confo (who may have a longer back with a weaker rear end -- maybe just out of shape?) usually bail out of jumpers before getting higher than 3'6

He is four years old! He will take time to develop a muscular hind end, and I have seen multiple horses with long backs and smallish rumps comete higher than 3'6. There are plenty of holsteiners out there without huge motors. His ability to jump the heights will become apparent as he gets older and the bigger fences are put in front of him. My horse is long backed and his hindquarters are small and as my Trainer is fond of telling me he could jump a house. Seriously, even a dairy cow can jump 3'6 I seen it o_O