Hello, I am Dr. Peter a licensed Veterinarian. I would like to help. I need to ask you a few questions to be well informed and give you the best advise. There is no additional charge for information request.
1- What breed is your dog?
2- Has he had any trauma to the tail?
3- Has he been swimming lately?
4- Besides not moving the tail, any other symptoms?
5- Is he on any medications?
6- Do you notice any swelling or lacerations on the tail?
What you are describing is consistent with what is called: "Frozen tail, cold tail or dead tail." This is a syndrome that we do not know why it occurs. It has been associated with activities like swimming or exposure to cold weather, but, it can also occur in the absence of these conditions.
They usually get better within 2 weeks with or without anti inflammatory medications. I would recommend you have him checked by your family veterinarian to make certain this is a Frozen tail and not something else like a fractured tail. In the meantime, you can start him on Buffered Aspirin 5-10mg per pound body weight every 12 hours with some food. I have included a link below where you can learn more about a frozen tail. Follow up with your family veterinarian.
If you have additional questions I will be glad to answer them.