To do that...
1. Go here when logged in as your son: https://tickets.brightonandhovealbion.com/CrmDetails.aspx#eSRO_Crm_ActiveTickets
2. Check the Chelsea ticket
3. Click/tap "Change Price Type"
That's the easy one to deal with.
You can do a one-time upgrade on the ticket and any adult is able to use it - no need for them to be a member, and no need to pay the STH "sharing" fee.