Merlin? Is not a mind-reader. He is, however, very adept at reading expressions; you have to be to survive in the Courts where every tick of muscle could mean something different.

"I was wrong, Jo. I'm sorry. I am very safe and you can just relax."

Look, he's even going to lift an arm so Doc can scoot against him for safety purposes while they watch a Shakespeare adaption. Chosen specifically for the drugged man because he's a teacher, Merlin's hoping it catches his interest.