I think it's just because he is a Japanese. To most East Asian people, we often use wrong tenses because there's no tenses in our native language. This happens to people even if they were in the U.S. for decades. It's actually not about how long you've been in the U.S., it's about when did you start using English as your primary language. If they started using English (as their major language) when they were 20s, it is likely they would make mistakes about using tenses.