Why can I no longer send messages to my friend's iPhone?

I have several contacts to whom I am able to send and receive text messages on my iPad. One friend had some sort of issue with his phone about a month ago. I think he switched providers or something. His phone stopped accepting my text messages. He thought if I waited until later in the month it would again accept my text messages but it does not accept my messages, and he is unable to send me text messages.
His phone is an iPad.
What on earth could be happening? He says he is able to send and receive from others. I am able to send and receive from all my others.???

Thank you. Oh, by the way...how will I find any responses that may be posted???? Thank you. PS I do't know what it means where it says Trackback, nor do I know what, if anything should be typed into the dialog box...??