Really listen to your son. Reassure him that it is no big deal to you how he does on these tests -- you love him just as he is. But it is a big deal that he have appropriate educational experiences. The analogy that helps with some discussions is medical: "If we were trying to decide if you needed an operation, we would need lots of tests to see if the operation was necessary. The information that's collected helps us take care of you." If he is really dead-set against the assessment, back off. A private center where the time and setting are optimal may also help if you can afford this. Good luck.
Talking About Testing with My Child
Gifted and Talented Expert Advice from Mary Ruth Coleman, Ph.D.