Another thing I read from that is he was going back to a place that last time had a negative spin on it and he was there for weeks. And lets face it A vet clinic is a human place to hold an animal for a few days but a few weeks probably caused him a bit of anxiety to return worried that he would be left there again. Over all all the above suggestions are appropriate but it may pay to see if you can just take him in every now and again to sit in the waiting room for a bit then go home. Teach him he will go how from the vet again.
Just like a dog can have a positive idea of the vet they can just as easily have a negative view.
Best of luck with him