pumping supply issue

My son takes 3oz breast milk every two hours. I'm stuck in a cycle where I pump one hour before he eats. Early in am it takes me a half hour to pump 3 oz. by 2 pm I am only able to pump 2 oz after 45-60 minutes of pumping. I breast feed throughout the night (11p-6a) and pump after he eats over night which is only twice and have just enough to feed him his first bottle in the morning. I go back to work in two weeks and need to have a stash built up but i can barely (not even) keep up with his appetite much loss stockpile breast milk. AND I know he will be going to 4oz a feeding soon and I'm scared. HELP!!