No. Such topics often involve inappropriate and abnormal fetishes that violate moral and ethical norms, and they should not be promoted or written about.
He should first try to cover up immediately out of instinct. Then, he should quickly compose himself and apologize for the awkward situation. He could say something like 'I'm sorry, Mom. This is really embarrassing.'
Well, this is really embarrassing. For the son, he might feel like he wants to disappear in that moment. But he should know it's a normal part of growing up, just that it happened at a bad time. The mother should pretend she didn't see much and give the son privacy. Maybe later, if she feels it's necessary, she could leave a note or something to let the son know that she understands it's a private matter and that she still loves him no matter what.
The son can quickly cover up and then be honest about his embarrassment. He might want to ask his mom for some privacy and then later have a conversation about how he'll be more careful about privacy in the future.
He may initially be mortified. However, instead of running away or shutting down, he could use this as an opportunity to learn more. He can ask his mom for some privacy guidelines. And if he has any concerns or confusion about his body development, he can take this chance to get some answers from his mom in a more respectful and appropriate way. This situation, although uncomfortable, can be a starting point for better communication between them about puberty - related topics.
She should stay calm and have an open and honest conversation with him. Let him know that it's a normal part of growing up but should be done in private.
The mother should immediately withdraw to respect the son's privacy. And later, she can try to act as if nothing happened to avoid making the son feel overly embarrassed.