My supply didn't go down, but I have an oversupply anyway. I did notice just visually that it seemed to be less fatty - when sitting in a bottle, the fat at the top seemed to be less. When I queried and asked around various health professionals the consistent answers I got were:
Tehcnically, and ED should not affect the quality of your milk, (as long as you are eating enough) as your body will take nutrients from your stores or wherever if it cant find them from the food, and also women on the third world who have very limited diets milk quality does not suffer. I felt that the quality of my milk was suffering though, but that was just a hunch.
As for supply, they told me that if you aren't eating enough, yes, it will go down, so increase your water intake and snack more often including protein snacks as much as possible. Oh and stress and sickness is a major factor that will affect your supply. So try to get good sleep and time out where you can.
If you want to stimulate your supply put him on a bit more often and let him feed longer if you can....