I am sorry to hear you are worried about Mojo. My name is Rebecca. I am a veterinarian and will do my best to help.
It sounds like something is bothering him in his hind end/hips/spine: not jumping to get onto the chair, holding his tail down. The diarrhea could be from something he ate that he shouldn't, but I would expect that would have been bothering him before now if the stranger walked him 4 days ago.
If I could, I would take his temperature; normal would be between 101 and 102. I would check to see if his anal glands are full or impacted or infected (they are just inside the anus). I would palpate his hips and spine to see if anything seems painful.
If his anal gland were impacted, emptying them might help. If they are infected, he might need antibiotics. If he seems painful, I would start him on medication for pain and inflammation. The best, ***** ***** and safest pain meds are by prescription, but you could try aspirin.
Most of your description tells me he is hurting, especially not jumping onto low chairs, and holding his tail down. Dogs can be very painful, but not complain by whining or crying. Beagles are prone to back pain, and it can happen without an accident or injury, just out of nowhere.
If you try aspirin, give it with food. You could try baby aspirin or a low dose aspirin, one per 15 pounds of body weight, once to twice daily. Give with food, and stop if there is any vomiting. Do not give ibuprofen, Advil, or Aleve to a dog.
If he continues like this, seeing his veterinarian to have his back and anal glands checked would be the place to start. Let me know what else I can answer or help with.