Disabling the connection between iPad and iPhone

I appologize if this question has been asked a million times. I did try looking for the anser before I wrote this.

We have an iPad for the family. My husband has an iPhone 5 for work. We are set up with the same apple id because we share our music. My problem is that my son uses imessage on the iPad, but my husband gets the messages on his iPhone. My husband tried shutting off imessage, but he started missing texts from co-workers and clients. I tried to go into settings on the iPad and delete his phone number, but I can't. I do not have an iPhone, but my email is checked off to be recieving and sending the imessage texts, but for some reason my husband keeps getting the messages. Since his is a work phone, he cannot be getting texts from my son's pre pubescent friends.

Is there anyway I can untether these two devices without doing a factory reset?

Thank you