Be aware of what this will do to your reputation with other drivers. The naive will look at it as you ended or tried to end a career of another over a few texts.
Whatever happened to people telling others how it is. If he’s upsetting you and causing a problem, go find him, look him in the eye and tell him what the issue is and to back off.
Then if he doesn’t then you tell him that you’re going to take things further. Tell your steward and they will talk to him and explain senority. If that doesn’t help things then you have to consider who you go to next.
In 20 years I’ve seen this situation a handful of times and management will not help you. The company will likely see you as a problem also (liability) as well and the union is going to defend anyone who is walked out.
Talk to the guy. Have the steward with you and that should end things.