When he acts calm, take him out of the stall and walk him in the pen or arena. Then gradually introduce the lunge circle again, walking, and gently ask for a trot, because you must make sure he understands the reins are there to steady and keep straight, not stop him. Only on rare, rare occasions should one be shorter than another and then only the inside rein. This is not usually necessary with most normal horses. If you have leather side reins constantly check to see that one is not stretching out more than another by holding them up against each other and measuring the length.
Side reins can be dangerous to horses because they artificially hold the jaw.
The problem is that give is sometimes not enough. I've seen many a horse rear up and go over backward due to tightly adjusted side reins without much give or without any elastic at all which gives the horse no room to escape the pull. They can only take so much, which is why they should be adjusted loosely at first and very gradually, over a period of weeks or months, shortened up a bit at a time. Most riders' hands are not as steady and solid as the side reins and that's the beauty of them. They are so consistent they teach a horse that to evade the bit is futile and that the constancy is to be relied upon.