Get Back to Blonde

A: Yes, your son can definitely get back to blonde, but since he used a permanent hair color, it's going to take some work. There are essentially three approaches: lightening the hair, using a color remover, or simply waiting for it to grow out naturally.
If you decide to tackle this at home, I'd recommend starting with a color remover kit like ColorFix rather than jumping straight to bleaching. These products are specifically designed to strip out artificial color without being quite as damaging as the traditional bleach-and-tone process. Just keep in mind that most people find color removers leave their hair looking kind of flat and beige afterward, so your son will probably want to follow up with a light blonde toner to add some life back to the color.
For the lightening route, I'd really suggest heading to a professional salon. The same goes for color removal if you're not feeling confident about doing it yourself. Salon professionals have the experience to lift dark color safely and can assess your son's hair condition throughout the process to minimize damage.
There's also the simple solution that works great for guys with shorter hair: just cut it off and start fresh. If your son's hair is about an inch or shorter, he could have it back to his current style with no color in roughly two months. Sometimes the easiest path is the best one, especially if he's not particularly attached to the length.
Whatever route you choose, just remember that going from black back to blonde is a pretty dramatic color change, so patience and realistic expectations will be your friends here.
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See also:
Hair coloring
Men's hair and hairstyles
Temporary, semi-permanent and permanent hair color