Low milk supply

Help! I have an almost 4 month old son. EBF since day one. Recently went back to work about 3.5 weeks ago, 12 hour shifts, only able to pump 2(3 if I’m lucky) times a day. I’ve noticed a pretty significant drop in supply since going back and I’m struggling to make enough for baby’s bottles while I’m at work(I work 3 in a row, off 4). My little one has also not been gaining weight as well as he had been since I’ve gone back. I’ve tried pumping/feeding more often when home, drinking/eating more but I’m not seeing much of a difference. I was really hoping to make it to at least 6 months without having to supplement, but it getting more difficult with each day. What are some things I could do to increase my supply?