I am a pretty even mix between an ISTJ and ISFJ, but I think my ISTJ side applies here even though I most often resort to being a feeler. For the most part, I very much dislike being impulsive...and when I think I am about acting impulsively, my mind corrects me by reminding me how much I hated the aftermath of my last impulsive decision. I like to have a plan and stick to it. I think a thing to note and ask your brother about is how comfortable does he feel AFTER making the impulsive decision? If an ISTJ makes an impulsive decision, they usually regret making the decisions (sometimes even when the outcomes are positive) because it made them feel uncomfortable to do so (and we don't like being pushed out of our comfort zone). But as I said before, acting impuslively doesn't mean he isn't an ISTJ, I just wouldn't expect him to do it often and would not at all expect him to act impulsively on life decisions.