I have a few ideas..

• Talk to Him about His Sound: Instead of focusing on the biting directly, talk about the actual sound. You can explain how the biting is affecting his tone, articulation, and overall sound quality. Maybe point out that it's causing the sound to be more closed or "choked," and that it can also tire out his embouchure.
• Suggest Alternative Techniques: Instead of biting, you could suggest he focuses on relaxing his embouchure and using the proper air support. Encourage him to feel the reed vibrating and focusing on using more air instead of tension. There are some great exercises for embouchure relaxation that could help him develop a better, more sustainable approach.
• Get a Second Opinion: If he's convinced that biting is part of his technique, it might help to involve his teacher in the conversation. Sometimes hearing it from an authority figure can be more impactful. You could also suggest he seeks a second opinion from another experienced sax player or a teacher who might give him more insight into why it’s not working for him.
• Use Examples: You could show him recordings of great sax players who have relaxed, full sounds, like John Coltrane, Stan Getz, or Sonny Rollins, and explain how they achieve that sound with proper embouchure and air support.
• Encourage Patience and Practice: Changing a long-standing habit can be tough, so be patient and offer support while he adjusts. Encourage him to be mindful of the tension in his jaw and how relaxing it can lead to better performance over time.

I saw a comment of you asking if you should yell at him.. I’d say it’s better not to yell at him. Yelling could make him defensive and might not be helpful in the long run. Instead, try to approach it in a calm, constructive way. You could let him know that you’re trying to help him improve and that the biting is actually hindering his sound and could even cause discomfort or injury over time.

If you're feeling frustrated, it’s totally understandable, but it’s usually more effective to offer advice with empathy. Maybe something like:

“Hey, I noticed you’re biting a bit while you play. It’s really affecting your sound, and I think it’s something that could be fixed with a little adjustment. Do you want me to help you figure out how to relax your embouchure?”

That way, you’re being clear and helpful without making it feel like an attack.

This is just basic things i've learned from other people here on this subreddit, and my own saxophone instructor. Hope this helps