Time keeping

Hi, My 18 year old Autistic son really struggles with time keeping.  He's very private and spends a lot of time in his room, so I can't say categorically what the hold up is. He can spend literally hours getting dressed or putting on shoes. To me, a NT, he appears to be procrastinating,  but he won't (can't?) tell us why it takes so long. He misses big chunks of school - especially the lessons early in the day as he can't get himself up and ready in time. He says he doesn't need any help (he's newly diagnosed,  if that makes any difference!). Are there any strategies we can help him to put in place to encourage him to be better with his time keeping? I also think he's up a lot during the night, but he denies this. I'd really like to understand better how his mind works! Thank you.