Not such a big fan of summer weather

Hey everyone. This is my first summer with my boston terrier, Danger. He is almost 4 years old and he weighs 26 pounds. (First off, is that too large for a boston?). I am a little worried about the summer heat and how it will effect Danger. I go outside to clean out my pool and I am out there for less than 5 minutes and I put Danger in his fenced area and I have to stop what I am doing and let him inside because he is panting so much. Is this normal? And what can I do to prevent him for getting so hot? And also what are some signs that he is getting too hot? I just want to make sure that he is alright outside. Please help!!!