You could try relaxation exercises, meditation,and soft music. I have used a tape that has calming music and words suggesting that I will go to sleep etc.
I've also written down everthing that I might think about when I go to bed. This includes the next days activities and any worries that I have. After I'm in bed, when I start to think about those things I remind myself that I've written them down so that I will not think of them. Repeat a mantra that has meaning for you. By doing that you can push the other thoughts out of your mind. Some that help are "I am calm and relaxed" "I'm getting sleepier and sleepier."
Another way to keep your thoughts at bay is visualation. I've mostly used tapes that described beautiful places along with "you are getting sleepy now" sorts of comments. The voice is gentle and the background music is calming.
These things won't work immediately. You'll need to practice them over and over.
I suggest that one reason you have difficulty getting to sleep is that you're over tired which causes your muscles to be tense and your mind to go into overtime. Try napping along with your son. Are there some tasks that you could leave undone? At least for awhile. Then use that time to sit and relax if your son is awake. If he's asleep, you sleep also.
One method to relax is to squeeze all of your muscles starting with your feet. Squeeze and the release your toes, then your foot, then your legs, etc. Focus on the feel of your muscles. You even squeeze your facial muscles by scrunching up your face and then relaxing it.
A meditation to try is to pay attention to your breath. Breathe in and out focusing on the breath.
When your mind wants to wander, gently stop yourself and go back to focusing on your breath, your muscles relaxing, the guided imagery, the music.
You can check out a book at the library for other methods of relaxing. One good book is The Relaxation Response by Benson.
It does take time to learn how to relax. Don't be discouraged.
There are herbs, such as Valerian, which are said to help with sleep. I don't know if you can use them when nursing.
A warm cup of herbal or decaffinated tea while I read something boring helps me. So does a warm bath with epsom salts in the water.
Having a wind down time before you get into bed helps a lot. Most people can't finish their chores and immediately go to sleep.
I hope you find something that will work. Try one thing at a time for a couple of weeks or more. You could try two different ideas by using one in the daytime and a different one at night. If your mind is always working in the day time try stilling your mind with affirmations, singing, listening to music, journaling.