Is there a word for "a person who hardly understands things"?

I have one friend who has a hard time understanding things other people say. While everyone except him understands what someone tried to say, this guy still understands nothing. We always have to explain more and more until he can get it. What can we call him?

It's the same as the student who hardly understands what a teacher says while other students can understand it. (Does not mean he is stupid. He can do the exam well. But the teacher has to speak more than usual.)

Solution 1:

One can hardly answer this question properly without knowing why the friend is slow to understand. You've ruled out stupidity, but he might be hard of hearing, dyslexic, perverse, autistic, obtuse or dense (both of which verge upon stupidity), or daydreaming. He might also recognize several possible, plausible meanings where others assume a single meaning is evident, and may have difficulty getting the speaker to isolate a single scenario.

Solution 2:

So he's not stupid, but needs everything explained many times ... you might say he was 'slow to catch on'. This at least suggests that once he has 'caught on' then he understands as well as the average person.

The only single word I can think of to get close to this would be obtuse.