My 7 year old BOXER has serious eating problems.......please help !!?

I have a 7 year old male boxer. He has not eaten in weeks. I noticed this about 3 weeks into it that his eating habits had extremely changed. He would eat very little and then nothing at all, but still drinks water like everythings normal. The symptoms I am seeing are extreme loss of appetite, dried up nose, and loss of energy...he will not even eat a piece of cheese or chicken broth. I brought him to his regular vet around 3 weeks into not eating and he found no problem; blood exams, stool check, heartworm check (yearly exam). A week later he is still not eating and his weight loss has started to be noticable. I bring him to a nother vet to get a second opinion. They examin him and to make a long story short...they open him up under his abdomin and check him from head to toe.....no problems found...cancer, toys, obstructions, ect.. I am force feeding him through a syringe w/blended food (vet recommended). Vet is stumped..please help !! possible mental problem ?????