Suppose a particular person is well-known for being skeptical about the truth of just about everything someone tells him. That is, he requires stronger evidence (whether it be mathematical, logical, empirical, whatever) to believe a statement is true, than other people. Suppose, however, that occasionally he unexpectedly lowers his standards, so that for a particular statement he can be described as gullible or naive.
Is there an adjective to describe him on those occasions?
Writing something like, "Occasionally the usually skeptical Mike can be surprisingly gullible," conveys the intended meaning. But I seeking a word that would convey the same meaning in a shorter sentence such as, "Mike is being [WORD]."