How to explain to a younger guy that you don't like him back without hurting his feelings?

This is an awkward situation. A boy 4 years younger than me (I'm 23, he is 19) requested me to help him with academics recently. I helped him. Later he started following me on social media and both of us kind of have similar interest areas. He texted me about some shows we both apparently love a lot and we had few nice conversations. I was talking to him the way I talk to my younger cousins. However, he has recently confessed to someone I know that he likes me.

It feels weird and kinda illegal (he's an adult as per my country's standards). It is more uncomfortable because in few months I'm going to teach at a college and there I'll have students his age. Someone that young is like a child to me. How do I nicely explain myself if he talks about it to me without hurting his feelings?

How to explain to a younger guy that you don't like him back without hurting his feelings?
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