The phrase "he is anything but mad" implies that the person in question is not crazy. There are many synonyms that can be used to convey the same meaning. One possible option is "he is perfectly sane" which means that the person is mentally healthy and in control of their thoughts and actions. Another option is "he is of sound mind" which suggests that the individual has a rational and stable mental state. Other synonyms include "he is not deranged", "he is not insane", "he is not unhinged", "he is not bonkers", and "he is not batty". All of these synonyms convey the same message that the person is not mentally unstable or unbalanced.