As he is so out of date most vets would want to do the full course, which is two jabs two weeks apart, so make sure he is booked in in time for you to start the course.
Personally I never have adult dogs done after their puppy jabs, as I don't believe they need it and there can be problems caused through vaccinating when they don't need it. I am surprised the club insists on it, as I know a lot of people who do agility and obedience as well as showing who do not innoculate adults. After all, he is not a risk to other dogs, it is him who is at risk if he is not up to date. Also many vets now acknowledge that vaccinating every year is not necessary, every three years is fine but some also agree it is not necessary once done as puppies. I'd phone the organisers and ask them, and tell them that you prefer not to give your dog unnecessary innoculations. You could ask the vet to do a titre test, which tells them the level of antibodies in the blood, and if they are adequate, he would not need vaccinating.