I'm not an expert so....
This partly sounds like just a boy thing. Guys are NOT multi-taskers. They can not think of more than one thing at a time. Given that he has ADHD, is brain is already skipping around. The key is to get him to make use of his skipping around.
For the guys that I have worked with is that you have to show them how to take a large task and break it down in to simple, quick pieces.
Like writing a paper, outlines are key. If the teacher tells you to write a paper about french fries, my son is going to just start writing things down. What he should do is first write down what he knows about french fries in notes, and then organize them into something that makes sense.
Same with your son. Break up his time. Teach him to focus on something for 30m at a time. Take a break and then either change subject and come back or pick back up where he left off. If a paper is going to take 3 hours, and math home work is going to take 1 hour. Start on the paper. after 30m, change to math, after 30m change back to the paper, after 30m back to math. math is done. change back to the paper...paper is half done.
Good luck,
M.