He may have been nursing more for comfort; this extra milk was not what he wanted. It will be helpful if you read this article from beginning to end first to get an overview, and then return to the sections that seem to apply to your situation. Like the tide, there is an ebb and flow.
He popped off her breast to say, "Cause it makes the milk come faster. There are a variety of flow rate nipples available and a mother may need to try many to find the one that flows best for her baby. Perhaps his appetite is building and he wants more milk.
Mom can have a glass of water and a lactogenic snack. He may want to move back to the breast with less milk as his tummy fills.
He may want to move back to the breast with less milk as his tummy fills. Have a drink on hand, a book or a magazine, listen to the radio or watch TV. Babies generally feed less often as they get older.
As you get to know your baby and your milk supply, you can make educated guesses on what he is trying to tell you. The milk might even spray, especially if your breasts were firm when you started.