Biggy
Why can't your son buy when they go on general sale with his digital account?
The ticket can be printed off at home, or left to collect, or sent to your address?
When I buy for others I have to assign which needs the account number for your sons account which he should know and its on the MFC website when he signs in.
If you want to do non digitally, go to the ticket office and queue when the general sale window opens, they will need his name and post code/address or account number. Of course it could be a long wait and they may sell out.