Hi, my son also was like this. He's still like this in 4th grade! Medication helps him now, but it's a lot of work to support him.
In kindergarten, definitely ask for help from the teacher on how to get homework done. Try doing 3-4 questions on a sheet, then 10 jumping jacks, or 60 seconds of running around the kitchen, it's like a reward. Or give him a koosh ball to squeeze in his hand while you work.
Also try other methods for learning - while the school may give you paper worksheets, you can get him wooden alphabet blocks or flash cards or an electronic alphabet game. These other ways may keep attention a bit better, esp. when you try different ways on different days.
My son also went to see the school counselor during kindergarten, maybe 2x a month - she played games with him to help re-enforce the concepts of following the rules.
At home will finally implemented 1-2-3 Magic and it has made a big difference with both our kids. We got the books and DVD from the library - I could only get my husband to watch the DVDs.