The same way you take an interest in any other person.

Ask them what they like and why they like it. Make it clear that you would like to understand.

And absolutely do NOT react in any way negatively. No “that’s it”, no nothing, no “ew”, no disappointment, nothing. If he tells you he watches paint dry in his free time, ask him what color and if you can join him some time.

If your son already has a hobby it’s kind of easier because you can ask him about things he did and things he finds inspiring. If he’s doing performance or sports, you can watch and support him doing that? Shop for equipment together, etc… If he has a competitive, creative problem don’t pressure him and reassure him.

If he likes media, try to lend some of his and try to enjoy it. Usually, even with hobbies you dislike, there is a point where it’s somewhat interesting if you are motivated to learn about it. That does take some time though and you will need to invest that time. You can just weave in a quick “… and how is your [x] going”. That’s reserved for more distant relatives.

But also he’s 15 so none of that may work for biological reasons.