Maybe he stutters in general and not just to you? Sometimes it comes out if a person is nervous
It could be because he likes you, and is afraid of saying something stupid/embarrassing. He could just be nervous around all girls. Or, maybe he has a natural condition to stutter when he talks. You know him better than we do, so it should be easier for you to figure this out. If you like him, and you think that he likes you, invite him to go do something together. Go somewhere. You inviting him somewhere will loosen him up, and he will be more comfortable around you in the future.
You remind him of his maths teacher
Just go: TA TA TA TA TADAYYYYYY JUNIOR.
At least you know he likes you very much :)
he's clearly nervous around you... he should warm up if you talk more often with him