How to change my iPhone to send texts to a Windows OS cell phone?

My son had an iPhone but has recently changed to a phone with a Windows OS. Since he has changed, all my text messages are coming back as undeliverable. The explanation I got was that my phone is set so it went through Apple for other iPhones (Internet) rather than through cells. This doesn't happen with Android phones… Only his so far. What changes do I need to make in my settings to be able to text him?

IPhones manage text between them using iMessage. The iPhones are registered with the iMessage service, and if an iPhone is replaced with a non-iPhone without first signing out of iMessage, other iPhones will continue to try to talk to it via iMessage. Your son needed to sign out of iMessage before replacing the iPhone with an Android phone.

This doesn't happen with Android phones because they always send texts using standard cellular texting protocols, which is more universal but also costs a lot more money (iMessage is free).