Concern) Teeth whitening

The child's orthodontic treatment is almost finished.

He is a college student who wants to whiten his teeth after removing his braces. I heard from people around him that it can be costly and may have side effects. Even after the procedure, if he neglects to manage his diet, his teeth may become stained again, so he's hesitant to proceed. He uses whitening toothpaste, but it seems to have minimal effect, so he's unsure what to do.