Why does my IPhone keep sending iMessage?

My boyfriend had an iPhone but recently down graded to the galaxy s4 xD and every time I send him a text my phone still recognizes it as an iPhone (iMessage) so I have to click on it and send it as a text message every time I send him a text which is so annoying. Does anyone know how to fix this issue?

I'm not sure. Maybe it still sends it as a imessage

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