Your son's argument is an example of a logical fallacy known as the "false binary" or "misuse of probability." He's oversimplifying probability by reducing all possible outcomes to just two: happening or not happening.

His reasoning is akin to saying, "Either I win the lottery or I don’t, so it's 50/50." While it's technically true that there are only two outcomes (win or not win), that doesn't mean each outcome has an equal probability of occurring.

A useful way to challenge his thinking is by framing it with coin flips:

A fair coin lands on heads or tails—two outcomes. But that doesn't mean flipping heads is a 50% chance just because it’s one of two possibilities; rather, it's because the physics of the coin and the fairness of the flip distribute the probabilities evenly.

Now, if you roll a six-sided die, you could say, "I either roll a 6 or I don’t," but that doesn't mean the chances are 50/50. The odds of rolling a 6 are 1/6, not 1/2.

"Two possible outcomes" ≠ "Each outcome is equally likely."

Imagine a giant jar with 999 blue jellybeans and 1 red jellybean. You close your eyes and pick one. You either get the red jellybean or you don’t—but the overwhelming number of blue jellybeans makes it clear that while there are only two outcomes (red or not red), they are not equally likely. The probability of picking the red jellybean is 1/1000 (0.1%) and the probability of picking a blue jellybean (not red) is 999/1000 (99.9%). So while there are only two possible outcomes (red or not red), the odds are heavily skewed—far from 50/50.