Since its not persistent, there will be nothing to worry much.
But its better to be sure that he does not wheeze. Kids wheeze because there is narrowing of the airways. When they are not treated adequately with bronchodilators, such involuntary exhalations can happen- in an attempt to open up the airways.
Just have him checked by your pediatrician
when he has a cough
. It will sort out the issue.
Hope my answer helps you.
Wish your son good health.