Yes, I'm confused by his missing this one, too. He grasped it pretty quickly once we talked briefly about it (and was able to do variants on it easily), but there was something about it on paper that made him react like this: "WHAT?! I will just guess." This is the same kid who used to happily do problems like this for fun and who can figure out money math in a snap (i.e. with the same root concept, like at the grocery store).
I had similar difficulties when these types of problems were introduced in school. I could solve everything up to that point practically instantly; suddenly being unable to do so, I felt it was impossible, and that I had to quickly guess the correct answer. I think it took me some time of solving these questions very slowly and precisely to raise my confidence (ignoring my teacher's complaints that I wasn't keeping up with class, or even listening, because I was still working on problems from 5-10 minutes ago at my own pace). Once I could answer consistently correctly, my pace picked up, and I was eventually ahead of everyone again. It is probably important for your son to build that confidence, too.
I think I panicked because I felt I had to answer instantly. My existence was basically defined as the math genius who solves everything instantly, at that point. Your son's difficulties sound similar in a way. His expectations of himself are possibly unrealistic; thus, he panics.
