There's a boy at school (my best friends older brother) and he's two years older than me. several times I've had my back to him he's tapped me on the shoulder. He knows who i am, but we don't really talk and we don't really know each other. Why does he do this? Does he like me?
Hello again. Is that the same lad whom you're trying to impress?
Kinda difficult to interpret a gesture such as a shoulder tap via description, without additional context. But it's generally just a sign of friendliness or support, a greeting. As your friend's sibling, that seems a reasonable explanation.