Breastfeeding struggles..

Help me please.. My son is 5 months old. He was exclusively breastfed with a very occasional bottle of breast milk until 14 weeks old, but now that I'm back to work he gets a bottle regularly with pumped milk. He was nearly 9 pounds at birth, however he only weighs just over 15 now. He's dropped from 75th and above percentiles in height and weight to very low below 25th percentile. I saw my lactation consultant, and he only takes on 2.5 ounces at breast, whereas with a bottle he takes in 5-6 oz. (and I can pump 5-8 oz at a time while at work so it's not a low supply problem anymore) he gets frustrated and distracted at breast because it's not instant gratification. We have struggled with nursing since day 1 with a bad latch, a large drop in weight after birth, slow growth, and now we know he's an ineffective nurser. I'm struggling because I used to love nursing but now I'm up all night stressed, I can't get a letdown at night unless I use a pump, and my baby isn't growing enough.