Suppose you ate an infinite amount Ice cream you could assume you would then weigh an infinite weight and ta da you managed to eat your weight. The other alternative is that he weighed nothing and did not eat any ice cream.
A biology major might point out that you would only retain so much weight from digestion so eventually the weight of the ice cream you eat could approach and surpass your actually weight.
My guess is that he just meant the boy ate a lot of ice cream.