If he's just 2 1/2 and he already knows his neck reining... was said to be "broke"... I'd hazard a guess that he was "broke" too fast and needs some holes filled up in his training. Unless he was started as a yearling, I just don't see how anyone could claim a 2 1/2 yr old is "broke" ... that term, to me, means a horse who has "done it all".
If he were my colt, I'd be putting him away for a couple more years before starting to work him under saddle. In the meantime I'd be starting him over, from square one. Go through the whole process of ground work, teaching him pressure from the ground, ground-driving him (with the bit), saddling etc... until he was totally fine with it... THEN I'd put my butt in the saddle.
2 1/2 is still a very young horse... here, I don't start horses under saddle until 4 - we take everything up to backing slowly starting as early as I get them (weanlings are taught to lead and move away and into pressure - other than that they get to live with the herd and grow up, yearlings start learning about the "odd" things I'm going to do with them - and live with the herd to grow up, 2yr old start lunging at a walk/trot and get used to tack, 3yr olds will be ground-driven, and then as 4yr olds, when they are more physically and mentally mature I get up on them and proceed from there at their own pace.
I just find training goes way smoother doing it this way (I used to "break" at 2.... found out I was making more work for myself by trying to work through the peak growing years - it's really hard to teach a horse to be correct, balanced and carry extra weight if they keep going bum high... so now I wait until the peak growing is over, and anytime a horse is noticeably growing they get time off)
I really don't think an issue like this should be treated by simply "wearing the horse out a little" first. Your horse is telling you he has a problem, he needs you to find out what it is - and fix THAT... not just wear him out enough that he can't complain anymore.