Whatsapp – new owner of my old number sees all old chats?

Hello, I got a new number and haven't deactivated the old number on WhatsApp yet.

I was logged out of WhatsApp overnight and a new profile picture was stored under my old number.

He had written in our family group that he had all the old chats, contacts and pictures.
I then asked him to delete the account and create a new one, but he refused. He laughed his head off at the pictures and chats.

Is there anything I can do about it, or could it just be stupid and he doesn't have any other chats and only the groups?