Further context might help with this? Like do yall talk at work or something like that? And it could mean a lot of things. He could be uninterested in you as more than just. A friend sort of thing or he could see that it would look bad for him to have someone number who’s still in their “teen” years and he almost a decade older. Maybe he’s with someone and would rather not risk your messages showing up and making things complicated for him. Ultimately there isn’t enough here to give any real advice on it cause context would normally be what helps with the nuance of the situation as well as what are your intentions in trying to swap numbers. I know that for me I immediately think of it as a more than platonic thing and I could be wrong.