“It might not even be his ID”

That is true. You should ask the lad his birthday before you ask for his ID. If they don’t match, it isn’t his.

“ID cards can be replaced in about five minutes at any district office."

They can if they are one of the newer sort with the electronic chip. If not the lad must return to his home for a replacement.

Obtaining a replacement isn’t the point. The point is that the boy knows when they find your body in the room his identity will be known.

“I don't want to sound naive, but has anyone ever heard of any Thai guy ever robbing a farang in his hotel room with a knife?”

Yes. It happened to me in my condo.

When a boy grabs you from behind and holds a sharp knife at your throat you can become very cooperative.

I know of no foolproof method of guaranteeing one’s safety. If anyone does I would surely like to learn it.