Re: to supplement or not to supplement?
Until a year, supplement with formula. After a year, use solids and water or animal milk (or juice, but make sure it's diluted with water, since pure juice is really not good for baby's teeth and it's always better to eat the whole fruit than just drink the juice). It sounds like you are nursing plenty when you're with your son, so you probably could get away with offering solids and water, but I think to be on the safe side you'd want to use formula.
I know you've tried changing pump parts and taking vitamins and all that, but have you tried the most effective supply increasers for a pumping mom: pumping more often and pumping for longer time periods?
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"