I can't tell you a whole lot about the fishing on GTB in June..I fished up there in July last year and struggled....I know there are some big ones in there, I just couldn't get it done...
Elk Rapids is an Awesome place to launch and you can slip back in there at noon or whatever and walk into town and get lunch...The water at GTB is like fishing in the Ocean, it is super clear and blue..It can get rough with a North or South wind, but that isn't a problem just hit one of the great inland lakes a short distance away..
I am going back up there the last weekend of June..There is a ton to do in the area..Boyne Mtn has a great waterpark that your son would love if you have the time...
GTB is definetly one of my favorite places to be, I just have to do a better job of figuring out the fish...