if you stop nursing at night, your supply will diminish at night. as a general rule, its supply and demand. the times when you keep the same schedule of nursing, your body will continue to produce at those times. if your schedule is a little erratic right now, you might want to throw in an extra nursing or pumping session for a while, only because stopping nursing overnight is a huge change, and things seem to change a lot around the 3-4 month mark anyway, so if it was me, to be safe, i would try to boost my supply right now since both of these things are happening at the same time, you are having a double whammy to your supply right now. i think that if you can get in a pumping overnight (dont wake that baby!) it would help a lot,,, but if you just cant see doing that, try to drink more water and get in another session during the day, or you can pump after he nurses for a while. increase your water, eliminate caffiene, and add some real oatmeal to your diet, it helps. add some maple syrup to it, it helps too, the fake cheap kind, it has fenugreek in it. an extra pumping session in between or after you nurse will help, do it a few times a day, even if you dont get anything at first, it will stimulate your supply.
it seems like you are doing a great job and know what you are doing, but just to be sure, are you finishing the first breast before you switch sides? generally around 20 min, though this time will get shorter as time goes on. if your baby isnt getting the fatty hind milk, he will be left hungry and he also wont get the fats that are essential for brain development. the same goes for pumping, you can see the color change, be sure the fatty hind milk is getting into each bottle.
best of luck, D.