Hey, first you might wanna try longing your horse w/ pessoa or vienna reins... this will help him build up the muscles in hte correct places without worrying bout us on their back
... at first keep them on hte long side until he/she gets used to the pressure and gets the idea....
Also, when you're riding, have your horse on a normal contact and then (starting at hte walk) kind of push with your hips forward and slowly let the reins slide out of your hands so you encourage your horse to streatch forward and downwards, keep this stretch for a couple strides and then slowly collect him back. As he becomes used to it and has more muscles, you can encourage him to be stretchy for longer periods of time. This also works with trot and canter, but make sure he doesnt get on his forhand.
Good luck and hope I helped :)