Circles, Circles Circles!!!!! This is all that I have ever done to get a horse to "set". but you must make sure to do them correctly. sit tall look in the direction of your turn, and apply pressure with your inside leg directly at the girth, and your outside leg behind the girth. This keeps your horses hindquarters from swinging out of the turn. This will get your horses whole body in a frame that will be much better to ride in. Also you said that your horse gets stiff at the trot and canter. I would do a couple of things. First while going straight on the rail walk for a few strides- then halt and back up. then immediately trot off. again for a few strides then halt back and trot off. This will get your horse working off his hind quarters and not the fore. Another great thing to do is supple the neck. When you are sitting in the saddle just chilling ask your horse to touch his nose to your boot. it can be a long process and you'll probably meet a ton of resistance but the pay off is well worth it. say you're asking for him to bend to the left. pick up the left rein and gently pull back toward your left hip bone. his head will flip and he might not know what you are asking but just hold gentle but steady pressure until his nose turn just a little bit toward you. then release and praise the snot out of him. Ask again until you get a little further. if he is a fast learner then by all means try for more but since you said he is young just ask for a little and let him soak it all in. Do that on both sides until he can touch your boot on both sides. This is all that I ever do with my youngins and They set up beautifully not from aids or ques but because they have the flexibility and respect for the bit, and it just goes natural for them. Good Luck!