Supplementing with formula

Danielle
My little guy is 8 wks. I exclusively nursed the  first four weeks and then did that along with pumping to bottle feed. Around 2 weeks ago, my baby wanted to eat way more often than what I was able to produce for him. When I pump am found every 3 hrs I only get  2 oz total so have never been able to build a supply and I've tried the teas, oatmeal, lactation cookies, water, etc.  
Anyways I started supplementing each bottle feed with Similiac for supplementation.  He eats about 3-4 oz every 3 hrs. But I'm only able to mix in 1 oz breast milk with 2.5-3 oz. Recently  he's screaming hungry around 2 hrs.
I don't want to give him much more bc then he spits up but wondering if the Sumuliac I'm using, since for supplementing, is not keeping him full and ms be I need to switch.   Does anyone have any advice or tried other kinds of formula mix with breast milk?  I fear my supply is almost up and soon  I will doing all formula