How to increase milk supply?

I’ve got three boys, and my first two I didn’t try to supply milk for or even breast feed just do to personal situations at the time. But my youngest little boy was born 2months early at 30.5 and is currently in the NICU still, they told me he was past the age for donor milk, and he wasn’t stable at the time to know if he could handle formula, so they told me my milk was his best chance for the time being. Not sure if they said that, just to get me to breastfeed, or not, but I’ve been hand pumping and hand expressing my milk since I’ve had him. He’ll be a month old tomorrow on the 10, and usually I can get anywhere from 40ml-60mls through out the day every pump session is every 3-4hrs. I prefer to send pump because it’s more convenient than waiting in place to use an electric pump. But I sleep through my midnight pump session and I pump at 5am when my husband gets up for work, and I can usually get about 60-80ml. This morning I barely got 40mls, and my 9am session I barely got 30mls. I try my best to keep up with drinking water, I’ve got a huge smart water bottle I refill atleast 2-3 times a day if not more. But I noticed yesterday my supply was a little on the lower side, and then today it’s really on the low side. Usually by the 3 1/2 - 4 he mark my boobs are hurting and heavy,but they haven’t been doing that today so far. I’ve tried the lactation cookies from Walmart,but I honestly can’t even tell if they’ve been helping over the last few weeks or not. I need helps or ideas on what I can do to boost my supply??? I go to the hospital every other day sometimes every 2 days depending on the weeks schedule,but even my normal supply amount is only enough for like 2days if that, so I want to be able to keep up with supplying my baby for his appetite. He’s eating about 41mls every 3hrs and his feeds are extended by a pump that goes for 2 hours, due to some issues he’s having.