I would do the bloodwork. Even if you can somewhat manage the symptoms via highly manufactured food, you still want to know if there is EPI, which can get worse as the dog ages, as well asa B-12 deficiency and SIBO (those are typically test for at the same time). Be sure you do the fast for the epi test. Vets who are hesitant to do necessary bloodwork are as bad as vets that overdo the testing!
I am not a fan of the force feeding of food that they don't want to eat. I trust the dog's instinct. As for changing from the food that he was on to the new fan, I can appreciate the hesitancy, no comparison in quality, though the content may be better either in digestibility or removal of an offending incredient. It could be as easy as a fat intolerance (hence the need for enzymes).
You need to find out why the other food was intolerable - ingredients, epi, sibo.