When did he start acting this way? Has it been fairly recently or has it been a gradual thing? I would think he needs to go back to the vet, either the one he has or another one, for a complete Wellness checkup. Including blood work and urinalysis.
I recently had to have one of my sweeties pts at less than three years old after trying unsuccessfully to get him healthy again. he had liver issues that may have been ok had I got him to the vet a little earlier. I implore you to find a vet and have this done. You do not want to have to go through the pain I have gone/am going through.