Why doesn’t Huck tell the Welshman the truth about the “Spaniard” at first? How does the Welshman figure out that Huck is holding back?

Huck is afraid that his name will be revealed and that he will be in danger.

“Please don’t ever tell I told you,” were Huck’s first words when he got in. “Please don’t—I’d be killed, sure—but the widow’s been good friends to me sometimes, and I want to tell—I will tell if you’ll promise you won’t ever say it was me.”