Spent my childhood snorkeling in the Bahamas, most of the time on my own, or on weekends with a friend, never once saw a shark
Also spent a summer working as a snorkeling instructor in Mexico, did dozens of snorkelling trips in Sea of Cortez with guests, never once saw a shark
You'll be fine but just in case are you confident you can swim faster than your son?
I'd be more worried about getting sun burn than being attacked by a shark