iMessage to an iCloud user keeps trying to send as SMS

My son has an iCloud account, and in my Contacts, his entry has his iCloud email and iPhone phone number in it. When I use Messages to contact him, it often defaults to trying to send an SMS to him instead of using iMessages. The SMS never goes through.

Oddly, if I manually select his phone number, it goes through Messages and works fine. But if I select his iCloud email address, it goes green like an SMS and doesn't work.

Why wouldn't using his iCloud email address route through Messages? How can I fix this?

Thanks!