If you're chatting with a Chaturbate model from Colombia or thereabouts (highly likely on the sex chat sites), know that WhatsApp is how they converse with almost everybody down there. They're ALL on it.
My boyfriend and several other friends are from there and it's just the way they carry on a conversation if they're not face-to-face with you. Or with their mom or their buddy on the other side of town.
So don't automatically assume there's something nefarious about it. If it was a perfectly legit desire to just chat with you and get to know each other better, he'd STILL be using WhatsApp to talk with you.
He probably wants to get away from chatting with you via the Chaturbate system because that's monitored by the company.
So simply asking to get on WhatsApp is not, on its own, a reason to think this is suspicious behavior.