Does your cat go outside at all?
Does he seem unable to raise the tail at all?
Have you checked under the tail for any sores or mats?
Do his eyes look normal?
It is posssible he got some damage to the tail such as a pull on it that has hurt the nerves in the tail though a painful mat might also make him keep it down. You can see if trimming the mat resolves the problem (just be careful when you trim as cat skin cuts as easy as butter with a hot knife).
If he is fearful of buzzing insects (usually learned when a cat gets stung) he may have had one show up unexpectedly when he was walking by it such as a fly or bee on the floor that suddenly took off buzzing as the cat approached. That might make him look closely at objects on the ground.
It could be he hurt himself in a reaction to an insect that scared him too and that is impacting how he holds his tail.As he goes out there is potential for intestinal parasites. What the vet looks for in the stool is microscopic eggs and oocysts as its rare you see a worm in the stool.
Stress can make the tail be dropped, illness, or injury so I'd suggest a vet check up again. The vet can compare this exam to the last one and may be able to find an injury or problem that can be resolved to help your cat feel better.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care
Thanks for the update - as you never mentioned food I'd never have suspected it!
Pretty much cats process meat or fish proteins the best as their digestive tracts are very short. Soy can cause gas which may have been botherng him!