I am sure this answer is not particularly useful anymore since I assume, given the delay, that he is no longer clenching his hands. If this has occurred more than once I would have him checked by your pediatrician who is hopefully a developmental pediatrician with expertise with autism. Ruling out medical issues you have probably encountered an unusual form of stereotypic behavior which I would try to redirect to handling some toy materials to see if you can get him interested in other sensory experiences.
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