Dog has a cold?

I have a 10 month old Yellow lab boxer mix. He is close to 70 lbs. We took him to his dog sitter who has plenty of acres for him to run outside. He even ran through the creek and pretty much was active for the 9 hours he was there. We live in upstate newyork so it is quite chilly. Starting on friday we noticed he was hacking and when he would have something come up it looked green like flem. Its hard for him to breathe but his nose seems pretty wet and he isnt acting out of the ordinary. He is eating his food fine. It has been better today and he only has started his doggie cough when he runs. What is it and what causes it?