My dog can't walk very far, and that's a problem help!?

I have a 4 year old shihtzu and I need to take him to the vet and the groomer. The distance isn't too far away but it's hard to take him out because after a few minutes of being outside he stops walking. I don't have a car so I'd have to take the bus and walk a block but he gets nervous on buses and starts screaming. He has a doggy bag but he starts panting insanely in there and walking is out of the question. It seems like I can't take him out when it's too hot too cold or too windy. What's wrong with him? He gotta get exercise! What should I do??