I'm from PA too and my dogs are not having problems with the heat although we do have air conditioning and when they come in they can cool off. Plus they love water so we have them hosed off several times and they even have their own kiddie pool! However, a 1 yr old should not being so lathargic. It could be something simple like worms. I had a similar bout with my first Golden and that's what it turned out to be.

Take your baby to the vet along with a stool sample.