I'm sorry it's taken me so long to get back to you.
There are a number of things that could be going on, but considering his age and the history, I would definitely be concerned about parvo. Intestinal parasites can also cause vomiting and diarrhea. Also, some pets have adverse reactions to metronidazole - including nausea, vomiting and lethargy.
If you feel he is so sick that he may not make it through the night, you should definitely get him into an emergency veterinary clinic in your area. If nothing else, they will be able to test him for parvo and rule that in or out, and evaluate his condition to determine what care he needs immediately.
You said you were in Dallas, TX. Here are some emergency veterinary clinics in the Dallas area. If one is not close to you, you should be able to call them and ask where the nearest one is.
LBJ / Greenville AveXXXXX Ste 118 (at Markville)
Dallas, TX 75243
Phone: (972) 994 -9110
Bush Turnpike / Central (75)
401 W. Pres Bush Tpke, Ste 113
Richardson, TX 75080 Phone:(NNN) NNN-NNNNbr />
North Texas Emergency Pet Clinic
3337 N. Fitzhugh
Dallas, TX 75204
(214) (NNN) NNN-NNNN
1712 West Frankford Suite #108
Carrollton, TX 75007
phone: (NNN) NNN-NNNN
Good luck with your pup. I hope it turns out to be nothing serious. If you have further questions, I should be online for a little while longer.