Since you say he works fine at home, I don't think a calming supplement is going to be effective for you. It's normal for a horse to be nervous and excitable when taken somewhere new, especially so if they haven't been off property in 8 years and you take them to a busy environment like a show. To get him over it, you have to get him used to going places. At first it could be trips to nearby stables that allow haul-ins, don't even worry about working him there at first- just put him in a round pen with some hay or hand walk him for a short while and then take him back home. Work up to taking him places and doing serious schooling sessions, then small show environments, etc.