Congratulations on your pregnancy.
To a certain extent, you're fighting biology to try and boost supply during pregnancy as many mothers experience this drop. The best thing - as usual - is to try and pump more often and get more out. I don't know whether this is feasible for you at the moment, though?
There's no reason your boy needs cow's milk, really, so any other would be fine if he'll take it. It's just a supplement to the rest of the diet if that makes sense, rather than a central source of nutrition.