Yes, I think a low supply like yours can be built up. What I'm not sure about is why it is low. Well, what I mean is that your supply is directly related to how much your breasts are stimulated to produce milk. Because you don't have as much milk as your son needs, you supplement with formula via the SNS. Using the SNS is supposed to help stimulate your breasts, but some babies figure out how to just suck the tube and not suck the breast. Is it possible that your son is doing that?
It's discouraging that all of the LCs you've contacted seem to have little experience with an SNS or LactAid. Have you considered asking your local LLL Leader for the names of LCs who could help?
In my first post, I said that you had two issues to address, but it looks like there is only one! That's good news. Since your son latches and sucks well at the breast, you do not need to focus on that. The challenge for you is increasing your supply (which requires stimulation via baby or pump, and possibly galactagogue(s)).
You're working hard for your baby!