Though Obi held distaste for his father he felt pity for the lion who promised to make him his heir. Viewing this as his dad wanting to gain his admiration the cub went along with it not knowing of what Scar had planned.
Scar: "You're going to be a prince Obi"
Obi: "How?"
Scar: "Oh you'll find out soon enough..."
Obi: "What about Lina?"
Scar: "Don't worry about your sister, she's no longer important. She's not like you"
Obi: "Right... If you say so dad"