SMS Text Messages to an android phone all of a sudden sending as imessage to unknown number

My dad has an iphone xr. He has been texting my brother at the same number for months. They go through 'green' as text messages as my brother has an andriod phone. All of a sudden a few days ago they started going through as 'blue' imessages. This new person is very angry that he keeps getting messages from my dad. My brothers number has not changed, I can still text him just fine. We cannot figure out how this happened and get it switched back. Any help would be greatly appreciated.