How long has the dog been losing weight? Breathing hard is NOT a good sign.

What does your Vet say? A Vet needs to see the dog in order to make a proper diagnosis.

Unfortunately none of us here can diagnose your dog. We can only guess at what might be wrong. Your dog sounds like he is in distress, it really is time to seek help from your vet.

The combination of sudden weight loss and difficulty breathing in an indication of several things, such as diabetes, heart, or kidney problems, or parasites such as the mosquito borne condition known as heartworms. All can often be treated, but all will continue to worsen if untreated. Unpleasant as it may sound, a vet needs to examine the pet, probably run a few tests to better outline the problems, and then make suggestions as to what may be best for your dog.