Unsure how to deal with kid I tutor

I 22F have been tutoring this 15 year old kid for a year now. He pays attention, listens to what I tell him to do and applies it instantly and honestly works hard. He used to be so shy that he wouldn’t even tell me about any questions he had or the grades he got but things improved after a week and he got a lot better.

Problem is that today it almost felt like he stopped taking me that seriously. He kept trying to joke around and using some like immature phrases (nothing crazy just things I imagine he wouldn’t say to his teachers). He is a good kid so if I tell him to stop he will listen but I worry about being too harsh with him. If he gets upset it would not be good for anybody. I tried to shut down any side convos but he kept bringing up really random things so in the end I just said that he can chat with whoever he wants once we finish but not me because I’m here to teach him not chat with him (sounds less bad in my language but still).

I feel really bad but idk what I’m supposed to do.. I even rely on this job for uni expenses and have a good relationship with his parents. I also genuinely want to see him improve so I don’t really like the way things have turned out.