I'm pretty sure this is Rhododendron maximum, the North American native rosebay rhododendron.
A broad-leaved evergreen, the rosebay rhodie is a big open shrub that can get 15' for sure, if not 30' over the course of it's lifetime for us down here in a Zone 6b or 7a. These guys are hardy to USDA Zone 3, which means well into Canada, where the species is apparently from. They usually bloom earlier, in June, but perhaps this guy was a new planting and therefore blooming a bit later. In any event, big fabulous clusters of white flowers are tinged with pink and yellow.
This guy is obviously just a youngster, weighing in at only 6-7' high and wide, but you can see the open habit and love for high shade or a little bit of sun through the canopy.