Reasoning behind my answer is , most susceptible are fast gaining calves on fast growing grass, his grass should be dorment this time of year, also he has them in a pasture now and is not worried, so by moving them to another pasture that should not make a difference. Also he said give them a booster shot , by that statement I take they have undergone previous shots.
The recommended procedure for vaccination is: Calves should receive 2 doses of vaccine before calves are 6 months of age. The 2 vaccinations should be 1 month apart. A booster vaccination 12 months later should give a lifetime immunity to blackleg. So to go by the book they should be vaccinated.I do not know of anyone in our area that gives the booster but that does not mean their are not some that give a 3rd booster.