I have never had Matthew tested...really need to I guess.
You could administer the test yourself. You can spread it out over several days like they do in schools. I got the test booklets and stuff from Bob Jones University. They send the test answer sheets and the booklets with the questions. Matthew would read the questions and mark the answers on the test sheets. When you're done. you send the whole shebang back to BJU, they'll grade the test and send you the results. I did both the Iowa Test of Basic Skills and the Cognitive Tests with Clare and Joseph.
If he's never done one of those 'bubble tests' before, you can just make a couple of copies of one of the answer sheets and let him practice filling in the 'bubbles' so he can see how easy it is to both fill in or erase, if he marks the wrong one. He'll be less nervous taking the test if he understands that. I think it is less than $50 for both tests and the grading service.