6 months old and supply not increasing?

bean6190

i thought the more you nurse, the more milk you produce, but my son is 6 1/2 months now and it seems like no matter how much i nurse him my supply never increases.  i pump twice at work and consistently got 8 oz total from 6 wks until about a month or so ago.  i already felt like this was barely enough, but now i can usually only get 6 or 7 and sometimes 8 oz.  

i feed him all the time while home.  when i feed him from the breast, he does great when i feel full, but then is hungry like 1-2 hours later.  my boobs don't even feel like they've had time to replinish!  and he gets frustrated that there is not enough milk.  when this first started happening i thought it would just take time for my body to get the message that he wants more, but it seems like it never has.  what's the deal?

he is very small still.  at 6 mo check up he was 12 lbs 15 oz, 24 1/2 inches! the doctor said he's not TOO worried, but wants him back in a month to do a growth check since his growth has slowed down.  if he hasn't picked up by that time, dr. will check into growth hormone issues.   

also, he is never not interested/will never turn down milk when offered bottle or breast.  is that normal?  i fell like maybe i should be giving him some formula if my milk is just not cutting it, but the doctor says to wait and see.

sorry this is so long.  i'm a worried mama!