You can book the sea otter for him for sept and Salamander for winter. In case he could not pass the lower level, you can cancel it and get the credit back.
BTW, kid learning skill is very strong, if you provide enough time for their swimming practice.
My daughter started her swimming class since her 3 years old. She can pass each level without any difficulty. Now she is just 4.5 and studying in Whale group. Since we brought her to 1 or 2 hrs public swimming after her swimming class. Previously she was very scared of water and now she enjoyed water.
Sometimes, I thought kids skills/abilities are really amazing than we expected. They made our life full of surprise. Enjoy...