Why won't my iPhone let me send an SMS message?

My father recently got an android after having an iPhone. My iPhone won't let me send him an SMS message. It only tries to send an iMessage. I DO have my SMS message setting turned on, so I don't understand why this is a problem.

The imessage works on a data plan. You can try switching off imessage and it might help.
On the iOS devices, Settings > Messages
On the Mac, Messages > Preferences > Accounts > iMessages account > Use the Sign Out button (removes the Apple ID effectively)

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