Ibn Hajar al-Haythami was asked this question and he replied:
The correct view is that he has to accept it. The claim of some that the one who told him cannot be certain and that it is only a possibility, and that the certainty of his being taahir (pure) cannot be nullified by the possibility that he may have passed wind, may be refuted by the fact that if someone were to tell him that some najaasah (impurity) has fallen into the water, he has to accept that even though a similar argument could apply here. The reason is that even though it may be a matter of probability, in many issues of sharee’ah it has to be dealt with as a matter of certainty.
And Allaah knows best what is correct.