I thought PG was for "no kids under 12 unless accompanied by an adult" and 12 was for kids over 12 can see on their own, while 12a means if you are 12 you can see it but only with an adult, and if you are 13 you can see it on your own, as 12a is our equiv of a PG-13 they have in other countries......
I would ring the cinema and check Tiffany
No point dragging the lad out only for him to get dissapointed if they won't let him in to see it