To help your supply, you could up your water intake to a gallon or more a day (sounds like a lot, but you need half that just for you and half for milk -- drink 10 oz with each pumping -- only way I ever got enough:) ), eat more protein with each meal, drink a beer a day (dr's orders:) ) and you can also wake and pump 1x during the night. Also, where do you pump? I worked fulltime and pumped for 2 by using a dark, cool, quiet room without distractions, and sat on a chair where I could lean forward and down between my knees a bit to use gravity, while expressing from back of breast to front to get the milk from the back flowing front. Letdowns worked much better with those combinations, and I did have to change up my environment and positions a bit every few weeks to continue getting 18-24 oz a day (3 pumpings) -- I figure it's like exercise and you hit a plateau:)
75% of nursing moms supplement with formula -- I supplemented 25-50% daily from 6mos to 15mos during the day, and still nursed successfully at night and on weekends without production troubles. If your son's like mine, at 6mos, they were taking 24-30 oz total during my 10hr workday, on top of 3 meals and 2 snacks -- he is old enough to introduce cereal, if that's the upset and fuss factor.
Try removing the Reglan and see if the fussy goes away in 3 days, and if not, perhaps just a gassy time -- babies and kids are definately moving targets:) Little brand makes a GREAT gas drop, or Mylicon if that one doesn't work. Plus, you can also try the bicycle with legs, and colic position holds (mainly belly across thighs or upside-down "football) to help with gas (once took my daughter to Childrens and was nothing but extreme gas -- argh!).
You ARE at a crossroads, both with your production and his growth -- you CAN be successful, just experiment a bit and find what the new factors are:)