hello and thank you for using this site to help you. Dogs will keep their tails down for a few reasons. The first being they are scared, but that does not show discomfort of the tail area so we can rule that out if there is discomfort. The other reasons if there is pain within the area would be a problem with the anal glands, which are located on either side of the anus, or dead tail. If the dog has had a bath or gone swimming lately that can cause dead tail and it affects the tail with pain.
I would check the anal area first, any swelling or discharge from the glands then that is likely the problem. If there is swelling or discharge you will need to have them looked at.
Cold tail or dead tail is painful , almost like a sprain for humans.
You should have the dog seen for that as well but you can try aspirin for both problems to alleviate the pain until you can have the dog looked at.
If there is generalized pain over the whole body as you mentioned when he lays down it can be a tick born disease such as Lyme, or it can be arthritis.
cold tail, dead tail http://www.thelabradorclub.com/subpages/coldwatertail.php
Anal glands http://http://www.marvistavet.com/html/body_anal_sacs.html
Aspirin and dosing. Be sure to use regular aspirin and not Tylenol or Ibuprofen
1 baby aspirin for every 16lbs or 1 adult aspirin for every 32 lbs, every 12 hours