Being an ottb, I would put my money on it that he is not using his back yet, thus making trying to sit on him even harder. Ottb's have a not so great tendency to work with all legs and no back. They may well be tracking up and have nice paces ad feel soft in front, but they are masters at holding their back stiff as a board. If the back of a horse is stiff, and the horse has already got a naturally 'bouncy' trot, your job of a rider is made very hard.
Before you worry about sit trotting him, you should be focussing all of your efforts into getting his back soft and swinging, with enough strength the carry you rather than bracing the back and perching you on top.
Once he develops a good, solid back that moves and swings with his gaits, you can start thinking about sit trot. Sitting trot is difficult for young/green horses that have not built up the muscle over their backs to carry the full weight of the rider, so as anebel said, it turns into a vicious circle. The horse has a stiff back so the trot is very bouncy. The rider tries to sit that trot, which is near impossible, thus bouncing around and thumping down on horse's back. Horse braces even more against the discomfort and possible pain of rider thumping around on it's back. Rider is thrown even more out of the saddle due to the brace and so it continues.
Unless the horse has a strong, soft back, sit trot will be extremely difficult and you will also be punishing your horse in the back by bouncing around. Never, ever, punish a horse in the back! You want their back to 'grow' and come up under you, not to suck away from you.
As for leaning back with gripped legs to sit trot... interesting way of coaching :/
Leaning back, first puts you behind the vertical, therefore behind the motion of the gait, put you out of balance, and unable to influence the horse's way of going with your totally non-effective seat. Gripped legs, simply deaden the horse's sides, tighten your upper leg and hips, thus tightening and restricting your pelvis. When everything is so tight, it is physically impossible to allow your body/pelvis to follow to motion of the horse, while controlling each step with your seat. You just cannot do it, and I am surprised that an instructor has told you to do that.