Thanks for the quick reply.
In direct answer to your question : This sort of thing is always a gamble, if you want to be sure you are doing the right thing you should have your vet check your Shih Tzu
over without delay. That said, while I cannot diagnose your dog's problem precisely via the internet I can tell you the following :
1. I would say from the fact that he is shivering and lethargic that it is highly likely that your dog is running a temperature and so may be in the grip of some kind of infection. As this has just been going on for a couple of hours and he is only two years of age the chances are that this is nothing too serious at present.
2. I have had many years experience of advising the clients at my clinic in an out of hours setting. I think if I had received a call where the client's dog was showing symptoms such as your one is my response would have been something along the lines of the following.
A. As long as your dog does not deteriorate further you would be taking very little risk in sitting things out for 12 hours or so to see how things go. Of course if things get worse you can review this decision and see a vet sooner if that is possible.
B. In the meantime keep your dog comfortable and reassured and try to keep him eating and drinking.
C. I am aware that some owners dose their dogs with aspirin when they have a slight fever and report success. On a short term home remedy basis aspirin would be much safer and more effective than the other human over the counter pain killers. A typical dose rate would be 5 mg per pound of your dog's body weight twice daily. The judgment whether to dose with aspirin is of course up to you but here is a link to a web page with all the facts you might need including contraindications : LINK
If I have not covered your question fully enough or you would like to ask more I will be online for the next hour or so and I will be at your disposal.Scott Nimmo BVMS MRCVS