Whats going on with my dog while he's walking?

My dog (3 yo, newfoundland, border colliemix) has never been a fan of summer, but he's starting to behave strangely. We walk him at the coolest point of the day, and no more than a mile,at a moderate pace & a mostly in the shade. But he has started drowling excessivly(his whole mouth is super foamy) and peeing constantly (not marking) and really slows down and seems unhappy. He acts normal besides during his walks and isn't drinking more than usual. Do we need to see a vet? Are we just overdoing it, what should we change?