Formula supplementing--help

Hitno28

My DS was born 3.5 wks ago and weighed in at 9lb 11oz.  He dropped to 8.15 at his first doc visit and by 2wks he only gained back half the weight.  I had issues bfing at first but saw a LC and that helped a lot.  At our doc appt last week they told us to supplment with formula.  Begrudgingly I did until his doc appt this week where he weighed 10.2lbs.  I was so happy to go back to just BFing.  However DS was not.  I feel like now i just don't produce enough milk for him.  He feeds for 45 min and still wants more almost every hour.  I check after he gets fussy and I can't even express any milk on either side.  So we revert to giving him formula.  Any advice on how to get more milk and get away from the formula?