IPhone – Contact recently upgraded to iPhone. How to tell the iPhone to send them iMessages now

iMessage can also register email address (like your Apple ID), that's how it works on iPad and iPod Touch.

So it's completely normal to be able to send iMessages with an SIM-free iPhone.

As long as you are connected to a WiFi network.

Your brother's Apple ID probably still has his phone number associated with it for iMessage. The smoothest way to deal with this is to have him disassociate his phone number from his Apple ID.

Apple has a self-service web site that explains how to accomplish this:

• https://selfsolve.apple.com/deregister-imessage

If that doesn't work, this older KB article has a helpful section "Unlink a phone number".

If he has other Apple devices (Mac or iPad) that are using his Apple ID, your iMessages may be being delivered there. If none of those devices are available when you send him an iMessage, your phone will eventually offer you the opportunity to resend the message as an SMS or MMS.

Unfortunately you can't disable iMessage on a per-contact basis.