When you're asking him to do something, are you in the room with him and do you have his attention? No good calling from another room.
When you've asked him, get him to tell you what it is he needs to do now.
Give him loads of praise for each thing he does - you got your pants on, well done, what's next? You got dressed by yourself after I asked just once, that's brilliant! etc
Get him to take charge more. Before we leave for school in the morning, what do we need to do? That's right, put clothes on and do teeth, what else? Put shoes and coat on, well done for remembering all those things, do you think you can do each thing in turn now?
Make it more fun - I bet you can't get dressed by the time I've done x.
Also get him to think it through. If you don't get dressed in time, you won't have time to play in the playground before school starts / jump on that tree you like on our way / choose a CD to play in the car. Being late is rude to Mr / Mrs Teacher, we need to make sure we get to school in time, how do you think you can help?
and so on and so on ... It requires lots of fore thought and effort on your part, so you need to think it all through before the flashpoints usually crop up, allow extra time etc.