I guess I would just offer a few tips from my experience.
1. Make sure your son gets comfortable with the bottle before you go back to work. I made the mistake with my first child of giving my son a bottle a handful of times and not being consistent. I thought he would be ok, but right before I went back to work, at 4 months, he refused to take the bottle. Needless to say this was a little traumatic. Make sure you offer your son a bottle every day at a set time frame.
2. With my first child, I pumped in the mid-morning, lunch time, and mid-afternoon. I was able to maintain enough milk for my son for 9 months. At 9 months, I began a position where I was in a client's offices, most of the time, and had to stop pumping. However, by this time, my son was enjoying solids, and we still did night time and morning feedings until the time he was about 1 year old.
3. Pumping this much is hard work. You have to close your office door and make sure there is a lock. I cannot tell you how many times people know, and don't wait for the answer, and just walk in. I had no embarrassments, but be sure your work station is set up to handle this. Make sure you have convenient access to a kitchen. You have to wash your equipment after each pumping session. Be ready for the weird looks. No one likes to see your pumping equipment. Ensure you have a nondescript thermos bag to hold your breast milk. Nobody likes to see breast milk in the refrigerator. Bring your own dishtowels and cleaning equipment. The communal sponges they put in the kitchen are disgusting!
4. The good news is that your baby will be going more and more on solids. As they do this they need the milk less. As I recall I always sent in about 4 bottles of breast milk to day care, and then had formula available for the supplement.
5. I highly recommend that you be prepared to use formula to supplement. I know there are hard core people out there who want to give nothing but breast milk, but this is not realistic for most working mothers. Get your son used to the taste of formula. It is very different. Contrary to what some people say, they don't get confused, and always prefer the breast. Be sure they get used to the powdered formula and not the liquid samples. My son would only take liquid, and it is much more expensive, because that is what I gave him (those little liquid sample cans).
6. Bottom line - don't beat your self up over this. As you can tell with my first son I tried very hard. With my daugther, I took a much more low key approach. I only pumped at lunch time. This was enough to keep my milk supply up for morning and night feedings. I could also send in one bottle to day care. The rest of the time she got formula at day care. You do what you can to give them breast milk, but you have to keep your sanity.
7. Frozen milk - the stored supply did not work well for me at all. It apparently had a funny taste (perhaps from the freezer) and my son always rejected it.