If a school cancels I-20 of an international student, how long can he stay in USA after the date of cancellation?

There is technically no grace period.  If you have applied to change status from F-1 to H-1, your application to change will be denied because you must be in status in order to change status.  You would then have to leave and process your H1 visa at a US consulate.  The most serious consequence of an overstay, a bar from returning to the US, does not kick in until you have overstayed by 180 days.