Thanks for the additional information.
The shaking/shivering that he's doing may be secondary to pain/discomfort although fevers can cause it as well. If you have a thermometer, it might be a good idea to double check his temperature; normal is between 100 and 102.5. If it's high, then the list of possible explanations for his behavior include a tick disease or possibly an infection of some sort.
If it's normal, then I have several other thoughts as what might be causing his behavior:
1. He could be experiencing a problem with his back such as compression of the spinal cord by a disc or mass. Sometimes a thorough physical exam will help to detect such a problem but usually x-rays or more advanced imaging is necessary.
2. He could have pulled a muscle or experienced a sprain. This breed tends to be pretty active and if that's his lifestyle, then he may have inadvertently injured himself.
3. I've seen dogs with ear infections behave this way, too. You wouldn't be able to detect an inner or middle ear infection but typically these dogs are tilting their heads so this seems less likely to be his problem.
4. Oral pain or problems with the teeth might cause him to be uncomfortable but, again, seems less likely if he's eating just fine.
I'm not certain that this is an ER type of situation unless his behavior worsens and he either doesn't want to move much at all or he yelps quite loudly when you touch him; then a vet visit sooner rather than later may be prudent.
I would have advised Aspirin since he's not vomiting and is still eating (as long as he's not currently taking steroids or other nsaid medication) since the anti-inflammatory and anti-pain properties may be of benefit for him. However, your dose is a little on the low side for a dog this size.
I would have suggested 1.5 of a full strength 325 mg tablet at one time and give it twice a day, with food to avoid stomach upset.
My only hesitation about its use would be that it might interfere with what your vet would want to prescribe if he has to be seen but several doses should be fine.
I'd also avoid use of a collar around his neck if he's leashed to go outside for elimination purposes but you may have already thought of this.
I hope this helps and that he is feeling better soon. Deb