Can you fast to lose weight even when you’re very thin?

Fasting in the short term won't hurt him, but it won't help him, either.

When the body senses crisis in the form of starvation, which is what fasting is, it switches into survival mode and will conserve every last calorie it can, making weight loss slower and more difficult. Taking fewer calories than you expend in a day - this is the only reliable way to lose weight.

Fasting is safe. People have done it for millions of years due to scarcity and religious reasons. Even skinny people have enough fat stores to go for days without food. Search Alan Fung on YouTube to get the basics if you are interested.

Sounds like body dysmorphia and isn't safe. He's just as likely to lose other tissue as well.

I lost 4 lbs this week without even trying to

If you do not eat then your body will breakdown stored energy but muscle can also be lost. Have you seen starvation victims such as anorexics or photos from Holocaust survivors. They are skin and bone because muscle has wasted away.This is extreme and most people dont have willpower to starve when food is available.
Fasting has benefits and is not bad even for a thin person but hopefully he is fasting and not starving. If he is so thin why does he want to lose weight?
If he starves himself he can impact his metabolism negatively as well.