ohh well I don't know about that! If he tested negative then chances are he doesn't have an allergy just an intolerance - MPI. Of course these tests can wrong but I don't want to panic you.
How exactly are you doing the trial? 1ml and then increase over how long? With Olly tests it started a touched cow's milk on his lip , 15 mins later he had a drop on his tongue, 15 mins later a teaspoon, etc until he refused anymore - that was about 27mls I think but they like then to have a lot more than that to prove there is no allergy.
As he didn't have enough we were sent home told to reintroduce dairy slowly ie starting with hidden dairy for 2 weeks then cooked (30 mins minimum) for 2 weeks etc. We are really only on stage 1 (6 weeks in mind you

) and we are getting NWs with wind and eczema

We have stopped it again today to see if all these clear up and then will retry.
Posted at same time... I don't know the answer I am afraid. I put my trust in the hospital. It would be less severe to trial cheese first but then again it may take longer to see the symptoms. Being a mod here for a while I have seen 2 schools of thought - do it really slowly (like we are) or just go for it and see what happens. Sorry not much help x