I would definitely get him at least flexion tested. My good friend's horse was doing the same thing and it was driving her crazy. He would toss his head in trot and canter almost continuously. She put him out to spell for a couple of months, brought him back in and sure enough, the head tossing has stopped. The vet said that it would have been pain that was causing the tossing, as the horse had injured it's hip months before and although not obviously lame, it was causing him some discomfort.
Also get an instructor or similar person to check that his bit is not pinching the corners of his mouth, and that his browband is not too small that it is pinching his ears. Do you know how to accurately check for saddle fit, or are you going by your own assumptions? If the saddle is slightly too narrow it can be pinching his shoulders and back, which restricts his movement and thus he physically cannot step out - hence the head tossing. If it is too wide it may be sliding over his back causing pain or pushing on his wither - again, head tossing. It may also be bridging, check under the flaps with the girth off and also done up, is there any space down the side of the saddle between his back and the panels?