Hello, I'm Dr. Deb.
Thanks for requesting me although I'm sorry that I've been off my computer until just now.
I'm also sorry that your dog isn't feeling well. Unfortunately, there could be several reasons for his behavior; there wouldn't be just one particular reason for it:
1. It might be helpful if you could take his temperature to see if he's running a fever. Normal is between 100 and 102.5. If it's elevated, then possible explanations include: tick diseases or possibly an infection. If the former, then he should start to feel better within just a few days and act more his normal self by 3 days if given Doxycycine or Minocycline (the treatment of choice for the majority of tick diseases).
If some other sort of infection, then different antibiotics are usually prescribed based on individual preference.
2. Dogs with anal gland issues can behave in some odd ways and typically affect the smaller dogs as opposed to the larger ones. What doesn't quite fit, however, would be his age if he's never had anal issues in the past. Having said that, though, if he has an anal gland abscess, he might be reluctant to move and might keep his tail tucked under him.
If you haven't already done so, then I'd lift his tail and closely examine around his rectum for evidence of swelling or what might appear to be wounds. If present, let me know and I can provide otc treatment options.
3. Dogs with spinal issues where there's compression of the spine by either a disc or a mass can also behave this way.
A thorough physical exam or x-rays might suggest this sort of problem although an MRI is usually needed to confirm it.
If he's not vomiting nor currently taking any non-steroidal anti-inflammatory medication, then he could be given Aspirin which could help with pain, inflammation or if there's a fever. The dose would be 10 mgs per pound of body weight twice a day, with food to avoid stomach upset.
The only disadvantage to giving him this drug is that it might interfere with what your vet might want to prescribe if he has to be seen.
But a few doses should be ok.
I hope this helps and that he's feeling better soon but if he continues to behave this way, then a vet visit may be prudent. Again, my apologies for the delayed reply. Deb