Oh boy, lots of stuff to help you. But where to start?
First idea is that while food can be a problem - its probably not. Sleep however is a huge difference maker. If he is not getting enough sleep it will make all of his behaviors much worse. And unfortunately kids with ADHD can have sleep problems. I have a really long post here -
http://www.medhelp.org/posts/Child-Behavior/Im-losing-sleep/show/2077099#post_9846032
to another parent whose child has sleep problems (among other things). If you take the time to go through it you will find lots of suggestions and places to go if he also is having sleep problems as well as strategies for discipline.
Trying to change the behavior of a young child with ADHD is very difficult, but it can be done. I think that one of the best sites I have found giving specific recommendations of doing so is found here -
http://www.onhealth.com/tips_for_parenting_a_child_with_adhd/page3.htm
Also, I highly recommend that any time he tries to hit you he goes to an immediate 4 minute timeout. Be very consistent with this. The change will not happen overnight as experts say it takes about 3 weeks of immediate, constant intervention to get a change - but it will happen.
Finally, the book I recommend the most to parents with these kinds of problems is "The ADD/ ADHD Answer book," by Susan Ashley. Its about $12 on Amazon. It may be the best money you have ever spent. It covers discipline, doing homework, how to work with the schools, etc. It will help you with all your kids.
Please let me know if you want any more specific information. I have lots to offer. Best wishes.