If his breathing is labored in any fashion, he should be professionally examined. My first questions to answer would be:
1) Rule out heartworms with an antigen test
2) any abnormal heart/lung sounds. Primary cardiac or respiratory disease? If any abnormals, do chest X-ray and CBC.
3) if the chest sounds OK, is there any abdominal discomfort or musculoskeletal pain. Age of the patient? Arthritis? Injury? X-rays as needed.
Anyway, this is something where someone should definitely see their vet.