How to automatically switch from iMessage to Text Message for one recipient?

I'm working on a group project with someone. I had communicated with him in the past via iMessage. I've sent him numerous texts in the past couple of days and it sends as an iMessage. He hasn't been responding, which was unlike him. He finally texted me today (the project is due tomorrow) and said "if you've been texting me about the project, I don't think I've received them. I got a new phone and iMessage no longer works on my phone." He must have gotten something other than an iPhone.

I replied to him and it still sent it as a blue iMessage, said delivered and everything. I held down the message and said "send as text", which worked, but is there a way to get my phone to automatically do that for him like it does for my other friends without iPhones?

Added (1). Update: I just sent him another message and now my phone automatically sent it as a text. It must have realized that after switching the first two or something? Either way, it's working now.

IMessage on his old iPhone must have still be turned on. Tell HIM (it should be HIS old iPhone NOT your iPhone) to turn it off by going to

Settings > Messages > then turn off iMessage