Horses don't usually rear for fun when you're riding them, ex-stud or no. There's usually a pain issue or a poor foundation at work. My mare reared for a while when I first got her, and while partly it had to do with a saddle, it also had to do with the fact that before I got her, she had been a pay and go trailhorse and had consequently gotten some very mixed messages from unschooled riders. She was also thoroughly sick of being cowboyed around (having her mouth yanked on and being kicked in the ribs with no warning). Most of her issues were the result of poor riding and training, and the rest was pain. To straighten her out, I resolved the pain issue, then I reschooled her with lunging and ground driving to get her stop and go buttons figured out. Then I learned to ride with my seat instead of my legs and hands. Kicking and pulling works with some horses, but it frustrates others and is almost never a good training method, so check to make sure that this isn't what's going on with your guy. I'm not saying that it's you who's messed up his training, but if he's lacking undersaddle manners, you might have to go back to ground work until you've earned his trust and respect.