Undergraduate/Graduate level books don't have to be patronizing. I am sure there are some mathematical fields in which he is not an expert but he has always wanted to learn more about. He might be very happy with an undergrad/grad level book in those topics. I don't know if you can ask him or any of his colleagues/friends/family. But I, for sure, would be very delighted receiving such a gift. In fact, not just math, but there may be plenty of other types of books (fiction/nonfiction) like scifi, mythologies, Freud, or WWII history books which he would truly appreciate.
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