Play lots of games with him. Like listening to the sounds. Or following instructions like 'Simon says'. Speech therapists for children essentially just use games as a way to develop speech.
Besides what the other poster has said (which I agree with), the most important thing that you can do is to make sure that you understand ADHD and how to work with your child to help him. A good site for that is here -
http://www.rxlist.com/tips_for_parenting_a_child_with_adhd/page3.htm#rewards
I also highly recommend the book, "The ADD/ ADHD Answer book," by Susan Ashley. These will help a lot. Best wishes.
You son is still very young, it will take some time for him to find his ground and adjust his speech for his audience. He still probably assumes everyone can understand him because of the positive reaction he gets from his peers and family. example: Smiling, nodding, agreeing and excitement by raising their voices which people give off when they don't understand a child but don't want to ask him to repeat himself. Your son reminds me of my son and my son is ADHD and has a speech delay as well. Before I share how similar the two children, please understand your son is still very young, the changes you hope to see will come, it is a matter of time, and medication is not for every child.
My son made annoying sounds when he ate, when he played any sort of video or board game and he made endless sound effects when he told stories. My son is now 12 and still makes most of these sounds; however the groans are sparse or absent useless he is mentally very distracted, or annoyed by loud chatter.
My boy still talks a lot and at a very rapid pace. His conversations range from what he saw on t.v, family, video games, what he did - what he wants to do, or how well he did in school etc. I love it to be honest, he is fully of life. He believes we can hear/know what he is thinking.
Medication helped my boy slow his thoughts down and speech pathology is helping him make more sense out of his stories. He now has a ending to all his stories, and enough detail to keep the listener interested. I am very proud of the change but it did take a lot of time.