We're both seniors in high school / 18 years old. He is just starting to date another girl (pretty sure he still thinks it's unofficial) but he has talked / texted with me everyday for weeks now. I do like him a bit but I would never pursue it unless he was single. I don't respond to his texts unless it's a direct question but he still contacts me multiple times daily. I have no idea how to confront him and say that if he has a girlfriend, he should stop trying to talk with me on the side. Should I clear up whether he likes me before telling him to stop? Any advice on how I can fix the situation in a mature way?

TLDR I want a guy who has a girlfriend to stop flirting with me.

Just remember if you do this to another female. Someday another female will do this to you.

Just remember if you do this to another female. Someday another female will do this to you.
You must have misread what I wrote: I'm telling him to STOP. I don't want him constantly trying to talk to me if he's in a relationship.

." Should I clear up whether he likes me before telling him to stop"? Why would you want to clear up a situation before you tell him to stop? Sounds like you want your "cake and eat it too". Turn the situation around, you have a guy and some girl tries to inject herself into your affair. How would you handle it?

You must have misread what I wrote: I'm telling him to STOP. I don't want him constantly trying to talk to me if he's in a relationship.Sounded like maybe you were considering it. But that clears it up.
You can just tell him you aren't interested and walk away... its really hard and boring to talk to someone who does not participate in the conversation. Simple is better. And long explanation leaves an opening to respond or contest or attempt to sway you. As it makes you sound less sure. Short and sweet leaves no room for debate and makes you sound significantly more decisive.

Block him and end this whole thing. Then you don't have to speculate, or even reply. Then it becomes harassment. Doesn't matter if he has a girlfriend or not, if you want this to stop, does it?

With that said, he has just started dating, why do you consider dating, a committed relationship, unless they have. Dating is just that. In fact in dating, you should be dating several people, to see and find out how you like them.

There is no rule, in fact in general the rule is you date several people, do not be committed until things get serous.