I too have ADHD and am impatient (combined type, severe). Impatience is not an excuse for financial illiteracy. And a graduated income tax is not complicated. Deductions, credits, exceptions, etc are where it gets complicated. But if he thinks he’s losing money by making more money, then he’s stupid.

Reading your other responses, you’re right. Not knowing something doesn’t make someone stupid. Refusing to learn something when you find out you don’t know it, that’s what makes someone stupid. Willful ignorance is stupid.

At the very least, he should just admit he knows nothing about it and just take your word for it. Deferring to others expertise in areas you are weak is smart.