Have you contacted the rangers? Shelties are terrible food hounds (and cute) and he might be begging in the camping areas...
I think that is an excellant idea.If there is food,people,maybe,just maybe he has been spotted.And quite possibly been fed by someone.
I still say,even tho this is a vast area,surrounded by water,and even farther off interstates,he may possibly have been seen by someone.
Have to stay positive.Shelties are thinkers,and knowing Angus,he's very friendly.Contacting rangers,picnic area personal,campgrounds needs to be done.