I don't really view a text as much different from leaving a voice message. They both get the same point across on a phone. I can't see it as something someone would get upset over and I am sure he wouldn't stop working with you.
One thought - are you sure the number you have is a cell number? Could it be an answering service he checks in with, or that he has forwarded to where he is at? I'm asking because if it isn't a cell (even if it's forwarded to his cell) then he of course wouldn't be able to receive the text.