Any way you slice it, given Brad's own experiences from when he was Will's age, and Will's experiences now, I don't think it makes sense to simply take what would be a parent's normal reaction to "the big M", paste it onto Brad, say "oh, that makes sense", and be done with it. For Brad's reaction to match a typical parent's reaction, given the history up to this point...I think something else is going on that we don't know about yet. That's not to say Brad didn't freak out in the slightest...I'm just not sure that that's the only thing fueling his reaction.
Who knows, maybe this really is just Brad underestimating Will's ability to handle unwanted advances...but I dunno.