I am sorry to hear you are worried about Bo. My name is Rebecca. I am a veterinarian and will do my best to help.
Lethargy and loss of appetite can be caused by many different things. It definitely indicates something is wrong, but more information would be need to tell you if this is an infection, fever, internal organ dysfunction (heart, liver, kidneys, pancreatitis, etc.), or even ( possible so it has to be on the list) cancer.
Things like this often get better in a day or so, but since it has been 3 days I would start to try to find the problem. I would begin by taking his temperature with a thermometer (101 to 102 is normal), checking his gums (should be pink but since he is a Lab they could be black in places; pale gums are a worry) and a thorough exam: listen with a stethscope, palpate internal organs and lymph nodes, check reflexes, evaluate if he is dehydrated from not eating, etc.
Is he at least drinking water? Any vomiting or diarrhea? Excessive thirst? There are so many things that lethargy and no appetite can be caused by. He is telling you he is ill and does not feel good; I would get him checked out as soon as you can. Let me know what other questions you have.