Sleeping through and Milk Supply

Am I reducing my supply by not feeding him and just expressing at these times? I am getting up in the middle of the night to relieve engorgement and usually express about 100mls off one side.
Your son is much more efficient at getting milk from you, so you will taking less milk than he would at this time - but since you are expressing during the night as well you may be producing MORE milk - it's a supply and demand thing, if you're using 600ml a day, thats what your body will produce,

I don't want my supply to drop off (as it is I feel kind of empty in the afternoons).
It's normal to feel "empty" in the afternoon - this just means your son will be getting more hindmilk, which is produced after the foremilk (the milk sitting in your boob that makes you feel full is fore milk, and looks thin, hindmilk is thicker and looks creamy)

I know that the baby is the best breast pump around but darn it,. I just can't get him to spit it into a bottle to save for later LOL
I expressed first thing in the morning, every day, after I fed her for ages, so I had a good couple of litres spare in the freezer. (Sadly I had to throw most of it out because it wasn't used, better too much then too little spare)

ETA: I am feeling more tired now than I was in the first 12 weeks and wonder if I need to increase my intake now that he's getting bigger and requiring more goodness...?
You will need to be eating well, he will be drinking more in total, and your body has probably started using up your vitamin stores if you havn't been eating enough fruit and veg. Eat well, and look at getting your iron levels checked and maybe taking a good supplement.

I definitely found I got more tired and drained the older she got, I think it's coz if you are exclusively breastfeeding, bubs demands increase as they grow which takes more out of you. I found it eased up again once she was well established on solids, probably around 7-8 mths.