"It's since he cares about you. He doesn't want you to get hurt, not even a little bit."

At least, that's what Inigo is assuming here. Or maybe he's projecting a bit, since it's how he'd feel in that situation. Heck, it's kind of what he's feeling right now - not exactly excited at the idea of Senku inducing something like that on himself on purpose.

"He'd probably murder me if he knew I was part of anything like that." Inigo says, shaking his head. "And I'd like to live a while longer, you know!"

Look, it's just a convenient excuse compared to I don't want to do it since I'd worry too much about you, since Inigo figures Senku is more likely to understand Gen's concern than Inigo's own. Since he knows Gen better.