A.,
He likely needs the food because he's growing! It will get better, I promise! :) The consultant told me, when my son was eating over 300 minutes a day, that babies that eat that much THRIVE. Exhausting, yes, but worth it so much to see those chubby cheeks and that big smile.
You are so lucky you can pump that much for him - I didn't have very much luck at all, and mine didn't take a bottle (hasn't yet).
My suggestion would be to either try the next size up on him, and pad it with a cotton diaper in front (the kind that are reusable, that they call burp cloths now). Gives him a little more leeway, and you a little time between wash loads. If you need to worry about rash, use a piece of FLEECE between the cotton and his skin. It won't stick and will wick things away. My friend Cheryl gave that tip when I was worried about diaper rash with my boy.
The only other suggestion I can make is to start a load of wash AM and PM, and line dry during the day (if you're like me) and use the dryer at night. Small loads are less economical, but this won't last forever. If you are buying next size clothes for him, you can always include them in the wash to get them ready for him to wear too. I usually put my delicates in there, too, just zip the zippers.
Good Luck! Let me know how it goes.
M.