Dealing with an argumentative student

Hello! I hope this is the right place for this.

I'm teaching students and they get good grades (96% another student got) in their French writing, so I think I'm doing a good job. Doesn't mean I'm not wrong sometimes of course. I just want to help him get the best result possible.

I'm tutoring this student. He's a teenager and has a tendency to argue over things. I'm not sure why. I've done research and apparently it might be a power struggle thing. I think his parents are pretty authoritative, but I'd just be speculating.

Anyhow, I'm in a difficult situation. That student will write things which are not recommended or risky, odd turn of phrase, odd vocabulary, etc... I will mention that to him and he replies with:

But not recommended doesn't mean it's wrong.

But this source (random website) says it's true so it must be.

He'll the pull a random, unreliable source.

I then need to spend time explaining the difference between a good source and less reliable one but it only leaves to more counter arguments.

I've read about this and I'm told I shouldn't argue back as it'll never end. I've tried to tell him: ''This is risky, you'll likely lose points for this'' but he keeps the mistake in, which really bothers me as it doesn't help anyone and reflects poorly on my work as a tutor.

I'm not sure what drives him to act this way. I've read he might be insecure and to protect his ego.

I'd like to handle this without hurting his feelings of course. On the other hand, I find I'm swimming against the current and don't know what to do.

If I tell him he's made a mistake, he'll systematically counter argue. If I don't say anything, he'll keep making mistakes and I feel like I'm failing him as a tutor.

What should I do?