Age limit is 14 and yes you're right, forms have to be signed by guardian/parent etc and of course proper clothes and helmet need to be worn. Both he and yourself will fill out separate forms for him being a passenger. Only drivers fill out the one you can download right now, the passenger ones are only available on the day. Please dont list him on your drivers entry form, instead leave an extra 5 mins at sign on to fill out the form for him and sign it.
Yes signing up for club and for L2S is fine on the day too, all in all you'll need 10 mins at sign on for all that so please come with enough time spare.
Can recommend the "Sydney Motorcycle shop" (locations around Sydney, google it) for helmets. Not too expensive for the plain black open faced versions.
Any other questions just ask!