I quite like this article. It explains all the different "Gadgets" and what not to use and why. What you are asking about is half way down. http://www.google.ca/imgres?imgurl=h...&ct=image&cd=3
You've left off a lot of information. Such as , how old is this horse, is he OTT, training history, how long you've had him, when he first went up on you was the first time he reared. Also, could something of changed with your tack or the fit? Has his teeth been looked at lately? Have you considered getting a chiro or vet out to look for pain issues?
I would be addressing some of the other issues first before throwing a gadget at him. Possibly enlist the help of a knowledgeable trainer
Horse's go "up" for only a couple of reasons, the main two are pain or fright..