Why won't my iPhone send texts to a Samsung on a different network?

[Missymoo195]

I have an iPhone 5S on Telus, my boyfriend used to have a iPhone as well. Recently got a Samsung on pay as you go with Rogers I believe. He can text me and I will recieve them all, however he won't get my texts to him. I can text my brother (also on Telus, he has a Samsung) and everything works fine. How do we fix this?  

[Nighthawk]

Did he un-register his iPhone from Apple/iCloud? If not, your iPhone will likely be trying to send iMessages instead of plain text messages. Is the text bubble blue (iMessage) or green (SMS)?

Send him here either way: https://selfsolve.apple.com/deregister-imessage

For the first bit after (reboot your iPhone), if the messages are still blue, tap them and see if it gives you an option to re-send as a text. I had this issue and it's a royal pain in the neck.