I should add that some people are SO "snobbish" about iMessage that they would probably leave someone out if only one person forces SMS rather than a "natural" iMessage discussion.
Natural is SMS (or, more and more, RCS). iMessage is abnormal - it's an iPhone-only thing.
When they have to do (and all of them have to do it, each one individually) is send him an iMessage
marked as an SMS message. Then their phones will send him an SMS from then on.
It's not that
he's different, it's that
Apple is different (and proprietary). It would be a lot simpler for everyone if Apple allowed other manufacturers to use the iMessage protocol. It's not as if an Android phone can't, it's just that any Android developer who developed an Android app that could handle iMessages would get sued by Apple. Since they chose the non-industry-standard way of doing things (iMessage), they have to make the change to include him. There's nothing he can do to receive iMessages on an Android phone. The phone has no way of handling them and just ignores them. (It's like a human being scratching his trunk - if a stupid elephant can do it why can't we? Apple is the elephant.)