Very saddened to hear of his passing!
I took an undergraduate concrete course from him (back in the mid 1990's), and served as a grader the next year for that same course.
He used to dazzle students by showing up for lecture with virtually no notes. Maybe a single piece of paper and some pieces of chalk. He could lecture for two hours straight without ever looking at the piece of paper. Working example problems, performing complex derivations, and such all from memory. He's pause occasionally to explain concepts at times, but I don't recall him ever pausing to work out the math in his head.