The Unopened Crown Road Allowance is exactly what it sounds like - it's "Crown Land" that is simply "not-opened", meaning that no road is currently there BUT it's reserved in case, at some later date, the County (which has jurisdiction over the unopened allowances in their county) want's to either 1) develop it into a road or 2) deem it surplus and sell it off.
Adjacent and abutting Landowners have NO SAY in who, how, when, why or how ANY member of the public uses that "CROWN LAND". The County has jurisdiction and unless they pass a by-law on how the allowance may be used, it's "open to the public for any legal use".
(do people have to get permission to drive down the road in front of your house simply because your property abuts the street? Why would it be different just because it's not paved or hard surface?)
So the short answer is YES, he can hunt on the unopened allowance provided that 1) he follows all applicable laws/rules (hunting regs, by-laws etc) and 2) he does not trespass onto the adjoing/abutting properties even to recover game that was shot in the allowance that crossed the "line", without landowner permission.
Now, given that a unopened allowance is almost always only 66 feet wide there isn't much to play with.
The usual use for an unopened allowance is to traverse to other crown OR private land beyond the abutting private property and that is where most of the friction occurs (ie. atv's going past adjacent properties - land owners posting the allowance to try and keep them out etc.
The only sure way to keep "everyone" out, if that's what you are after, is to go to the County and make an application to buy the allowance. It can be a (somewhat) long and expensive prospect, but in the end you would own the allowance as well and you post everything.