Android user doesn't get my messages...

The phone will automatically send the message in the appropriate format.

I do not get an alert that message can not be delivered.

What does that mean? Does that mean that the SMS (green) messages get sent and you don't get an error message?

SMS messaging is a carrier feature so, if it's not working, the first thing you're going to need to do is to contact your carrier.

See also:

If you can't send or receive messages on your iPhone or iPad - Apple Support

Katrina B wrote:

In the past, an undelivered message to others prompted a "send as text" message option with an exclamation point. That does not happen any more.

That would only appear if an iMessage (blue) failed to send. In that situation, the app is asking your if you want to try sending the message again as an SMS instead. You won't see that with an SMS that fails. I believe you just get the exclamation point. The fact that you're getting no error message at all would indicate that the problem is most likely on the other end. As far as your phone knows, the message has been sent.

The non-delivery is intermittent. We will try clearing the messages cache on the Android next, assuming we can find that setting on his non-iPhone! :)

Ha! Good luck with that.