First, your DS is older than six. Second, being chronically late can just be a personality trait, I know many who are. But it could also be a red flag. It could mean parents don't care, there is no money for reliable transportation, there are other issues going on in the home, or it could mean nothing at all.
I myself was late very often in third grade. So much so that the teachers had a conference with me and my mom about it. My mom worked nights, and had only been in bed a few hours before I had to get up, and my dad was out the door super early every morning, which left me to get ready on my own. I had poor time management skills, and missed the bus on a regular basis. Then my mom had to get up, get dressed, and drive me to school. So I was late. Of course my poor time management skills stemmed from the fact that I would turn on the TV before I was finished getting ready. Lesson learned after that meeting.
But that was the proper way to address it. Speak to the kid and the parents. See what is going on, then work with that. Not punish a six-year-old for what is likely, though not necessarily, beyond their control.