Is your horse abit spooky on a hack? if he is then that may be why your horse doesnt want to go infront because he feels safer following or being behind the other horse, because my horse did the exact same thing so i started hacking out on my own alot more! going to more places and see new things and showing her things that spook most horses and now she will walk infront or beside another horse, but she does every now and then refuse to go infront as when i kick her to walk on ahead she will stop dead, but its progress, so try what i said, go on more hacks on your own (espically new places) try and make him look at things that spook him and show him things that may spook other horses so that he is less scared of things, hope it helps