See the Admit cards can only be provided by the School authority only. SO if you can visit the regional office/Study center of the Open School board your son is studying. They will provide you required forms and you have to apply in those in order to get a duplicate of the admit card. If the board is away from your place, then contact on the official contact number as provided on the official website of the Open school which you can easily find out searching on Google.

I hope this answer provides you some insight. Have a great eve!