Agree with SE that without more info, musclewise it could be just the way he is. I prefer not to use gadgets to train, and I think it's also important the horse is properly educated to respond to them properly before using.
First & foremost I'd get him & his saddle checked out & ensure he's not in pain. Hopefully all is well and it's not a case of can't raise his back, round up, etc, but physical discomfort/pain is often behind behavioural/muscle problems. Especially if you think his mouth/bit is a problem, either physically or due to previous training, I'd consider training him in a halter or bitless bridle, to begin with at least. Changing the tack and methods of training can help break those automatic reactions that are associated with them and reducing any chance of pain can also help avoid this behaviour/attitude.
You don't say anything about his behaviour, training, or about your training of him, so I can only guess, but I'd guess he needs to learn how to relax first & foremost. Calm, low stress training is probably something he's had little experience with as a racehorse.
After I'd established he understood how to reliably & softly yield to pressure, not just from the bit but in all ways, then I'd probably start long reining or lunging him, teaching him first at a walk, to go calmly on a loose rein, taking up rein pressure only if/when needed to change/control his movement. I would teach him this in the saddle too, before I would begin thinking about teaching him to accept 'contact'.
Making a practice of taking up the reins softly with only a 'contact' for a moment before asking anything of him with more pressure while teaching him in this relaxed way will help him understand how to accept and not resist it when you start teaching him with contact.