I also have Erb's Palsey in my left shoulder/neck. I am now almost 20, and I have had therapy and tests done up until I was 12, when I moved away from my therapist I never was able to continue occupational therapy.
Some of the things that helped me the most were wall push-ups, and a thing called 'superman' which you lay on your stomach, put your arms as straight out in front of you as possible, and lay your legs straight behind you. In order to do one set, you lift your legs off the ground about 4-8 inches, and lift arms about 5 inches off the floor in front of you all at the same time. Then after holding that position for 15 seconds, go back to the resting position. This should work mainly your back/shoulder muscles, your abs, and stretch the muscles that get tight when our Erb's palsey is being 'difficult.'
This is a great exercise for many of the muscles that are hard to target, and when doing it, you should work your way up slowly, and make sure you are ready to go for longer, so that you don't stress those muscles too much. If you are not safe about things like that, it can have a reverse reaction, where you get hurt, not better. I hope that I helped some. Have a great day! -Whit.