Thread Switch from MMS to SMS. How can I change it back?

Previously, my friend had an Android phone that I would text (green) using my Messages app.

He switched to iPhone on Thursday, turned on iMessage, and messages that I sent him were blue (iMessage).

However, yesterday, my messages turned green. I checked his phone and somehow iMessage was off - he may have accidentally toggled the slider as he set up the phone wanting iMessage on. I sent another message that ended up being green.

Frustrated, I open a new message and typed in his iMessage-enabled email. The resulting message is blue.

So: under his contact information, I have his phone number and email. In his iMessage, he selected to receive iMessage through both phone number and email. My question is: can I get future messages in the first thread to send as blue? I have a lot of history in the first thread and do not want to delete/scroll through my messages to find that thread again. I would like get that first thread to re-enable iMessage.

Background (not sure if the info below is useful in any way):

I had both his phone number and email saved in his contacts before he got an iPhone/Apple ID.

After he set up his phone: inside his contact card under LINKED CONTACTS section, it shows two contacts, one with his iCloud, the other with his Gmail.