There are many ways to go about the situation. No matter who it is that he is texting. Whether it be a Co-Worker, or just a friend, get your husband to understand how you feel about it but let him know you are not telling him what to do!
Get in his head. You have to listen to him, so that you can see where the problems are coming from. Ask him questions that get him to think. Don't make him feel like he is doing something wrong.
For example, ask him "Why don`t you text me during the day?" Express your feelings to him. But, be calm so he does not get defensive about the subject. Anger will get you nowhere! Negotiate with one another so it doesn't cause future conflict.