I agree to go to a 2nd vet!
Sorry I read this again, and saw you have taken him to the vet.
But I think you ought to try a second opinion from another vet.
You must take him to the vet to be checked.
I don't know what could be wrong with him, it is a thought that he could possibly have a blockage somewhere in his GI tract. Does he chew things? Toys? Furniture? Sticks or wood? Rawhide bones? Anything?
Or he could possibly have an infection.
Hopefully the vet can find out what is wrong. Take a stool sample with you, it might help in diagnosis.
The vet will probably run some blood tests on him, to see if his white blood cell count is up, or whether he has fever.
Do take him as soon as possible. I do hope he is getting better soon.