There is nothing you can do to prepare for this kind of test. It isn't knowledge per se, as much as HOW he thinks. Do not stress him out trying get him to do his best or anything or making the test out to be a big deal.
Just tell him they want to see HOW he learns and they will administer this to him to help figure it out.
There isn't doing well or not doing well, it is just HOW he figures things out. So do his best, but don't worry about it affecting his grades or anything like that.
He might find it fun if you let him.
And by the way, there isn't "passing" and "failing" with a test like this. It is a cognitive abilities type thing. They really will see how he thinks to some degree. What kind of spacial reasoning skills he has. How abstract he is able to think. Can he mentally spin pictures around on the page to compare them. These are things you can't really LEARN. You either do it or you don't.
Just don't make it out to be some big thing for your son. Let him go in and just do it and be done with it. That's all he needs to know.
I went through it. So has my daughter. We both were in TAG programs. I didn't study for it. I didn't even know I was going to be tested. They just pulled me out of class one day. My parents probably knew, but I didn't. That was 6th grade back in the day (25 years ago or so). I told my daughter, but we didn't do anything to prepare. I just told her they were going to do it. Then asked her what she thought about it afterwards. She got a kick out of some of the questions. :)