Join Date: Oct 2006
Location: London, UK
Programs: BA Gold, VS Silver, SQ Silver, IHG Plat Amb, Hilton Gold, Marriott Plat, Accor Silver, Radisson Prem
Posts: 16,872
As far as I know, T5 is a fully computerised system. There's no guest book. Guests can only enter with the Executive Club cardholder.
The boarding pass is scanned on entry to the lounge, so it will show that your son is not eligible for lounge access - both the boarding pass and card need to match.