Father just pointed out an error we made in his swim.
We'd worked out his feeding schedule ahead of time, based on feedings every 20 minutes. When we talked this over with some of his friends the day before, they said that that was far to often, and that we should do feedings more like every 45 minutes. So we took that advice.
Dad just pointed out that we forgot to increase the caloric content of each feeding. So his caloric intake was less than half what we'd planned for. No wonder he ran out of energy.