Switch from iMessage to Text Message

My husband got a new phone and switched from an iPhone to an Android. My phone thinks that his number is still an iPhone and he can't get any of my messages because it keeps sending them as iMessages. Is there any way to change this but only for one phone number (not all messages)?

I know that I can tap on each individual message and send as a text message but I was wondering if there was a way to default it, just for him.

Thanks for the help.