Should I take my dog to the vet, or does he need a haircut?

Last week, I took him to the vet to get vaccinated, and he was fine. Now he has a cold, which I'm very sure is unrelated. He has a runny nose and sneezes sometimes, but he doesn't cough. My brother took him out to walk, and after like, 10 yards, he refused to walk anymore. It's aroind 90-something degrees outside though, and since my dog doesn't have all his shots yet, i can't take him to the groomer's to get his hair cut. We he too hot, or is this because of the cold?