He needs to go to the Vet right away. It could be any number of things. Dogs tend not to show pain, especially Labs which would be pretty useless as hunting dogs if they whimped out at every little thing.
Because of the eating issue I am thinking blockage. Has he pooped?
When my Lab girl had an acute episode of invertebral disk disease and significant spinal stenosis one of the first things we noticed was she would only walk backwards.
In my experience a Lab that won't eat is feeling mighty bad. Dumb dogs, they wag their tails regardless.
I wouldn't put too much stock in the tail.
How wonderful of you to take in a dog at his age. Old dogs are special. Sounds like you are too.