Re: Not HTA. Disclosure question
Were there signs posted saying that alcohol was prohibited?
If there is some sign or notice that alcohol is not permitted, that's one thing, but...
LIQUOR LICENCE ACT OF ONTARIO
Unlawful possession or consumption
Definition
31. (1) In this section,
"residence" means a place that is actually occupied and used as a dwelling, whether or not in common with other persons, including all premises used in conjunction with the place to which the general public is not invited or permitted access, and, if the place occupied and used as a dwelling is a tent, includes the land immediately adjacent to and used in conjunction with the tent. R.S.O. 1990, c. L.19, s. 31 (1).
Unlawful possession or consumption
(2) No person shall have or consume liquor in any place other than,
(a) a residence;
(b) premises in respect of which a licence or permit is issued; or
(c) a private place as defined in the regulations. R.S.O. 1990, c. L.19, s. 31 (2).
Exception
(3) Subsection (2) does not apply to the possession of liquor that is in a closed container. R.S.O. 1990, c. L.19, s. 31 (3).
A tent is a residence according to the Liquor Licence Act. 
Okay, they can ban alcohol in certain places, but if your stepson believed he was acting lawfully and there were no obvious signs posting to the contrary, he might be able to get the fine dramatically lowered.
Dumbass former co-worker of mine had exactly the same thing happen to him a few years ago. He looked up the Liquor Licence Act section referenced above, called the Prosecutor, had a meeting and he paid a $25 fine. Or at least he says he did.