Without acceptance of the aids, there can be no collection, no connection, no using himself properly.
From your posts, I'm guessing that you're using "collection" to mean moving united, back to front, in a connected, organized fashion, not "collected" in the dressage sense, but even so, you're a long way away from either.
You mentioned you're working with a trainer, *with a trainer's supervision*, a program of lunging in a surcingle and side reins can be helpful in reclaiming an OTTB. It encourages them to work correctly and begin muscling their topline, making the undersaddle work easier later on. However, done incorrectly, it can just confirm their bad racetrack habits, so do work closely with your trainer.
CONTROL comes from acceptance of the aids, and correct application of the aids, not equipment. When you are working undersaddle, I would strongly recommend using elementary aids on loose reins, with voice commands, while doing lots of basic transitions. The voice commands and loose rein pull-release, pull-release will give you something that can overwrite the racehorse "Riders pulling - must lean and run faster" conditioning.