So he'd been using a dating app on his work phone? That's not very smart.
And then someone (possibly him but possibly not) gave you a 'new' number, but as you never had the old number you've no idea if it's actually a new number at all. And now you've moved off the app and on to WhatsApp. Do you know if you're even communicating with the same person as before?
It all sounds a bit scammery to me.