My 12 year old has started making his own 'protein shakes' after cricket

He has been watching videos of cricketers' diets. The result is a shake every evening after practice: two bananas, a large spoon of peanut butter, milk, and what I now know is three spoons of sugar 'for taste'. Some days a scoop of the chocolate powder too. I don't want to kill the interest. He is cooking something, he's reading labels, he's asking whether dal has protein, all of which I like. But this glass is basically dessert, and he drinks it an hour before dinner and then eats half his dinner. My partner says leave it, it's bananas and milk. I keep thinking the sugar and the timing are the actual issue, not the shake. How have others steered this without turning it into a lecture?