I think the only way to know for sure what's causing the problem is to go back to basics, but you must do what you feel is right for you. I don't think that there are any foods that are foolproof, allergy-wise - anything can cause problems for a particular baby.
I think I started out doubting that Kitty's problems would ever be as bad as say, Kelly's James' are, and I still hope they're not, having heard some of what she has gone through with him. But Kitty's problems have become a lot worse than I'd anticipated, sadly. I think we all hope there will be an easy pain-free solution to the allergy problems, and that if we just do the one 'right,' magic thing, it'll all clear up and go away, but it's not always that easy.
I knew Kitty had problems with milk, but had no idea that she was also allergic to dust, cats, horses...
I think your lo is small enough to not miss the solids as much as you think he might. I'd go for filling him up with the ground rice/Neocate mix instead for the meantime. It'd only be for a few weeks at the most, just to get his skin back to a good state. Otherwise, you're in the situation where the eczema's not clearing and you've no idea what's causing it, because there are already too many foods in the mix, with no idea which ones might be problematic.
I'll butt out now, because this is obviously just my view and I do think you need to make the right decisions for you at the stage you're at now. I do wish you loads of luck though. Keep us all posted on your progress, as I know we're all rooting for you and hoping your lo is soon all peachy again. Lots of love!