Not being able to pump much, or anything, means nothing, some women don't let down for the pump at all, for some it just takes longer before they get more. This doesn't mean your supply necessarily is low!!
I've battled low supply and won (although it took 5 months), and I can't let down for the pump. I just don't get much. Not even when DD was getting all from me, around 6 months, and gaining beautifully!
I realise that your supply is low now, it may not have been at first, despite not getting much when pumping, and despite baby not gaining well.
You may not have had low supply in the first place, but it may have been caused by something else. This paragrapgh is from the new edition of “The womanly art of breastfeeding”, LLL's book, and it describes babies like mine:
“In contrast, the baby who sleeps through most of his meals, which are usually long and slow, may be using numerous sucks to get every swallow of milk. It takes a lot of time and energy to eat that way, and these babies doze as much as they can. But take them off the breast and their eyes fly open open. They watch you with a worried expression and do their best to get back on the breast, where they doze off again. Some of these babies sleep really well at night, not from contentment but to conserve energy, and they usually aren't gaining.” p. 120
DD lost weight at 4 weeks, and the LC found she didn't swallow much, so we had to revise feeding, feed on a sort of schedule (to get as much as possible in her), I took fenugreek capsules and breastfeeding teas, and keep swapping breasts to stimulate the breasts to produce more milk and DD to feed. We needed to feed extra, and fed donated milk in an SNS. And ,y supply did go up, just not enough. Later, when she was 3 months old, my GP, on rec from the LC, and supported by DD's specialist ped, prescribed Domperidone.
I just thought from what you were describing, that something similar may have been going on with your child...
My advice to you to get supply up would be:
See if you can get a hold of a dr who will precribe Domp., or order it online
Get an SNS or lactaid, or make your own at breasts supplementer, using an ordinary bottle, with the nipple whole cut bigger, and a feeding probe (which you should be able to buy at a pharmacy, I think) put into the milk in the bottle. Then tape the other end to your nipple, and your child can get milk flowing faster and easier than from your breast. As your supply builds up and your baby takes the breast, you can add the supplementer at the end of the feed. If your baby won't take the breast you might try taping the sond to your finger first, so he knows that is the formula he is used to! You will also need a small syringe, to squirt water through the sond to clean it, apart from sterilizing it.
But I think getting your child onto the breast is important, if you can't pump. It is possible to get your supply up, still!