I think just properly accounting for wins and losses can be good at instilling a sense of humility; I'd recommend he calculate his track record if he hasn't already. I enjoyed the book Thinking in Bets by Annie Duke, which is about the psychology of developing some habits around making bets and the cognitive biases we have. A Random Walk Down Wall St. is also a classic that is good at instilling a sense of humility in you as an individual investor.