first of all. if hes your horse(??) its your job to school him. you have to teach him to go round, because right now he doesnt know any better, and hes probably really lazy because he doesnt respect yu? do yu let him go all slow without disiplining him? do you let him refuse jumps? do you give him tons of treats and let him do pretty much whatever he wants? thats how you lose his respect. and second, hes only six.. hes just starting to mature, you need to take time and teach him, if hes refusing hes probably trying to tell you something. maybe he's in pain, and if he was overjumped as a foal, theres probably a pretty good chance of it, maybe hes scared, or maybe he just cant jump that high.. with a six year old you have to train him and teach him, and now you may have to start all over, start with a crossrail and start jumping that. if he refuses a jump like that do not circle him, do not go around it. stop!!! like literally make him halt! backup to in front of the jump and make the horse jump it, even if he knocks it down of just steps over it, he must go over it!!! this is how you teach him not to refuse. and then keep doing corssrails until he goes over them 100% over the time, then after about a week of riding then raise the jump, do this ever 6 or 7 times you ride hin. itll take many weeks, but it is how you will get an honest horse that is trained to jump that high. take your time!