Thank you for your patience. There are a number of things that could be going on here with your boy this evening and we need to consider anything from an infectious gastroenteritis, to a toxicity (hopefully there isn't anything that he could have gotten into), to intestinal worms or even a foreign body obstruction. You will definitely need to continue to keep a close eye on him, and if this panting and lethargy continues, then you will need to get him seen by your local ER or after hours vet tonight.
For now, make sure he has plenty of fresh water available and encourage him to drink. The biggest concern right now is that he could become dehydrated given the diarrhea and lethargy he has. If he won't drink, then you are best to get him seen by your local ER vet tonight where they can start him on some intravenous (IV) fluids. You can with hold his food until the morning now, and from then you can start him on a bland diet of cooked, boneless, skinless chicken breast and boiled white rice. Don't worry about getting him to eat tonight, but definitely encourage him to drink now and over night. If he isn't interested in drinking, then you could try making him up a fresh chicken broth. For this, just boil up some fresh chicken until cooked, scoop off the solids, allow the liquid to cool, then feed him this luke-warm broth.
For now, please keep a close eye on his mucus membranes, capillary refill time and respiratory rate as follows:
Mucus membranes - flip his lip and look at the color of his gums. They should maintain a nice salmon pink color. Get him to the emergency Vet if they appear white or very pale pink, or if they are a dark deep red color.
Capillary Refill time - this measures blood perfusion and test this by putting your thumb on his gum to apply pressure. After you release your thumb you will see the gum blanch. Capillary refill time is the amount of time it takes (in seconds) for the gum to return to a healthy pink color from the blanched white color. If 2 seconds or less don't worry - if it is taking significantly more time, again - off to the emergency Vet.
Respiratory Rate - if he is continuously panting throughout the night, this is a sign of shock and or pain and a signal for a trip to the emergency Vet.
As he has been licking his lips, he is likely quite nauseous. As such, you may also want to try him with a little pepcid. The typical dose for this type of situation is 0.25mg per pound of body weight up to twice daily. You can read more about the use of Pepcid in dogs online here: http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid
Best of luck with your boy and hopefully these symptoms settle in the next few hours. As above, if he continues to pant, seems very lethargic, or if any of the parameters above don't seem right, then definitely play it safe and get him seen tonight. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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