I would always tell people that it isn't a matter of "smart", more a matter of passion. Basically that even if you THINK you lack the natural abilities that some people have you can more than make up for them with a willingness to learn. (story at end of post)
Also, a good point to always make, FIRST is about teamwork. I know very few people who would be capable of taking a FRC robot from design through to competition all alone. Yet I know TONS of "smart" people. You don't need to know how everything is done, you need to know what you have an interest in.
An interesting story about someone who feels they are not so smart:
My father is a man who often tells us he has never read a whole book in his life. I believe him, he is slow at reading. He is horrible with computers, and frankly gets confused using a TV. What my father IS good at is math, not so much the calculus math, he hasn't taken much past algebra. But he is very at using numbers to make a point. By looking at numbers he was able to help us keep our school alive. Here is a man who claims he is not very smart but using what he does have, and what he knows, he was able to make a HUGE difference.
Another point about my father, despite not understanding computers, he follows several tech stocks and surprises me with his understanding of what they make on a pretty regular basis.
I tell this story because here is a person who doesn't think they are smart (like your young man) who has made a massive difference for at least one school. Brains aren't everything, in fact they count for less than nothing. What matters is using your abilities, whatever they may be, to improve yourself and those around you. Oh and for reference, I think he is one of the smartest people I know.
Sorry for the essay
