What makes Kevin so angry his eyes blaze?

The answer to this question can be found in chapter 4. Freak and Max are still getting to know each other at this point, so they don't know all of the other person's idiosyncrasies, likes, dislikes, preconceived notions, etc. The chapter starts with Freak telling Max about King Arthur and the knights, and the more Freak talks, the more Max realizes that Freak is a genius level kid. The other thing that impresses Max is how passionately Freak shares his knowledge.

This is the first time for me, hearing Freak really talk, and right away I know one thing: When he’s talking, you can’t take your eyes off of him. His hands are moving, and it’s like he’s really seeing it, this story about an old king.

Freak starts telling Max about the knights and how the knights were essentially the predecessors to modern day robots. Max makes the mistake of saying that he thought robots weren't real and only existed in the movies. Freak can barely contain himself.

". . . which is sort of how they program robots right now.”
I go, “I thought there weren’t any real robots. Just in the movies.”
Boy does that make his eyes blaze. Like whoa! talk about laser beams! He’s like fuming, so upset he can hardly talk.

Freak is a smart kid, and he's passionate about a lot of things. Science, technology, and robots rank high on his list of passions, and he's a bit angered and dumbfounded at Max's completely erroneous thoughts on robots. Freak gets control of himself and then tells Max about how very real and very cool robots are.