Thanks for the additional information.
Based on what your are describing he sounds like some of his signs are checking out just fine though the shaking and the shallow breathing is worrisome. At this time he could have a problem in his stomach
or intestines that is causing discomfort or this could be coming from his neck or back. Because he is not eating
or drinking on his own very well, I would strongly recommend taking him to an emergency clinic tonight to make sure that he gets a thorough exam and possibly some other tests if needed based on what the veterinarian detects.
If an emergency room visit is not possible you can try to encourage him to drink by offering him some pedialyte or trying to get him to eat by offering him some boiled ground beef or chicken
breast and rice. I would feed him two parts rice to one part meat. This is very bland and also very easy for him to digest. If he does have an upset stomach
or a intestinal problem causing him the discomfort then this food should be very safe fro him also. If he does vomit then I would not feed him anymore as he could have something more serious causing him to feel uncomfortable. As long as he is not vomiting you can also give him some pepcid AC very safely. His dose would be one tablet or 10 mg every 24 hours. This can be repeated every day for at least 5-10 days if needed without any risks of side effects.
Overall though, I would consider an emergency room visit first because the shallow breathing and the lethargy is worrisome and should be evaluated tonight rather than waiting overnight to find out the exact cause. This way you can also start him on the correct treatment to make him feel better much sooner.
Hope this helps - please reply if you have any other questions.