The only way you can buy single use access is if your ticket was purchased on aircanada.com You can add access up to 24 hours prior to departure by accessing the booking and adding the MLL access. Not cheap though as it could be as high as $45 depending on fare class. $45 will go a long way at a restaurant in the TB area of the terminal.

I'm assuming that your son does not have a United Club or US Airways membership as both of these grant access to the MLL regardless of status.

Unfortunately, the only other option is to buy a year long membership (or sign up for a credit card that offers the access) which is probably cost prohibitive.

(I guess you can also ask your son to look for a kind person who can guest him into the lounge as well).