(((hugs))) and good luck with the appointment tomorrow. I will definitely be watching for your update!
When I did a dairy free trial for ds' eczema I was told that it can take up to 2 weeks to get the protein completely out of your system. Having said that, some moms notice improvement much more quickly, so it's certainly possible!
As far as the pumping... What I'm going to say is going to sound odd coming from a breastfeeding moderator, but is there a reason that you don't want him to have any formula? I completely understand wanting to ebf - we were ebf for milk until I introduced soy milk around 16mo when my pumping output dropped drastically - BUT if the dairy proteins in your previously stored milk are causing intestinal damage, surely *some* hypoallergenic formula would be a better option?
To put it into perspective, if your little guy got hives or swelling every time he drank your stored milk you wouldn't even consider continuing to feed it to him. The intestinal damage caused by MPI can be just as harmful, you just don't see it immediately. In your position, I truly would consider buying even a small tin of hypoallergenic formula. You could mix it with your breastmilk, just an oz or two (or however much you would need) per bottle feed, until you can build up your storage again. My hat's off to you for doing everything you can to ebf even with returning to work - this is just a little speed bump

And I wouldn't dump or donate your stash just yet - see the doctor, get some tests, and wait for the results. You never know.
More (((hugs))) xxxx Will be thinking of you tomorrow.