How old is he, and how is his hair cut/styled? His hair may get curlier as he gets older, or his hair may be cut too short to show any curl...

But also, I'm pretty sure it's possible! Thinking back on what I learned in Biology classes, straight hair is a recessive trait and curly hair is dominant, meaning curly hair will show unless both parents have the recessive trait for straight hair (as in, they both had a parent who had straight hair and passed that along to them). SO, in the right situations, yes, it can change that dramatically -- the genes just need to combine a certain way.

Now, I could also be completely wrong here -- I haven't taken Biology in, um, 5 years lol So other curlies, feel free to debunk what I just said.
Originally Posted by oh.hi.its.steph
He's an adult (21) and his hair is semi-long (~4 inch) but it still looks straight. He's not using any straightening products/tools because he had the same hair as a kid as well and as long as I know him.

As you said I think it's probably a rare genetic recombination thing.