The first step to improving ADHD behavior (in this case, distractibility, being off task, no self-initiation), is treatment. That could be medication or alternative therapies, but the ADHD needs to be addressed.
After that, it’s time to implement accommodations in the classroom. The teacher should remind him to pack homework and turn in homework. S/he should oversee that process to teach him lagging organization and planning skills. A daily report card is a common tool used for this.
Tools like the RE-vibe could be helpful too, as a reminder to check with himself and see if he’s on task. He may be a bit young to use that successfully right now, especially if he lacks motivation to get his work done or motivation to be on task. A periodic checkin from his teacher may be more effective right now.
There are many, many more possible accommodations — you have to request those that your son might find useful based on the particular behaviors you’re trying to change/improve. This free toolkit download from ADDitude will provide sample accommodations, as well as processes and resources for securing accommodations in school for your child:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism