From the original version of your question, I understand that the Apple Watch belongs to your son, rather than to you.
A cracked or detached screen can be covered under warranty under certain circumstances, such as when the damage is caused by expansion of a defective battery or due to a manufacturing defect.
Whilst I am not seeking to take sides here, if Apple has already physically inspected the Apple Watch and ruled out any causes that would mean the issue was covered under warranty, then I'm afraid that it seems like the only remaining possible cause is accidental damage, as cracks are caused by impacts of some kind.
Based on my experience of helping users with Apple Watch issues, I would only say that, if accidental damage is the cause, then your son would not be the first person - or the last - to accidentally damage the screen without being aware of when and how it happened.
In respect of any future purchase, you may wish to buy AppleCare+, which includes cover for up to two instances of accidental damage, each subject to payment of an excess / service fee. For example: