I might be able to add a bit more information for you in addition to what was mentioned above. In an older lab, there is something called Cushing's Disease that is very common. It is a disease of the endocrine system, and basically it causes the body to release more steroid. This steroid, among other things, causes these dogs to pant constantly and excessively, mainly because they feel hotter than usual (even if their temperature is normal). This can be treated, but it is typically expensive, so if you cannot afford it I would probably not do anything (if this is what is wrong with your dog).
Another cause for panting would be severe pain, which can be caused by spinal problems such as a slipped disc, or by internal pain caused by something like pancreatitis. These things would typically cause your dog to not want to eat, act lethargic, etc. So if your dog is acting totally normal, but is just panting, then I would guess your dog is not in pain, and it is more likely Cushing's disease. There isn't anything you can do at home for Cushing's disease, but the good news is that it isn't typically painful or uncomfortable, so your dog can continue to live with it.
Without seeing a vet, it is impossible to say for sure what the cause is, but I hope this helps. Once your dog stops wanting to eat, that is usually a good sign that he is uncomfortable and it might be time to put him down.
Best of luck.