I considered it a strike because she went from nursing 6-8 times per day to refusing to nurse totally - biting when I attempted to nurse her or turning away screaming, never latching. This was a separation-induced strike, too which makes it different. I pumped ~5-6x a day to keep up my supply, and it did dwindle slightly but it comes right back when the baby starts nursing again. This is my first baby so I am no expert, but that seemed too quick to be weaning to me, despite all the well-intended family advice. Like you said in your first post, i did lots of crying and feeling really depressed. Oddly, what gave me a bit of hope was that an IBCLC told me that they had seen nursing strikes for 40+ days, and I thought, ok well I will just do my best until I totally loose my supply, and I felt very determined to continue trying as we headed into winter sick season. Also, supply can drop but you can get it back, which was a big pressure relief to me. As long as you still have even drips, you can get some supply back. I also read that a baby only needs to ingest 2oz breastmilk a day for immunological benefits, so every little bit is valuable!
Aside from the health issues you are tackling, how are you giving the pumped milk to your son? I found that some sippy cups have a "bite valve" (OUCH!), and also giving my daughter anything where the flow is so instantly rewarding that she doesn't want to have to work so hard for it while nursing - both I think partially contributed to my nursing strike problem. You indicated your son could drain a sippy cup in a min flat makes me think he may be getting lots of instant gratification. Thermos brand sippy cups hurt your wallet, but the baby has to do some work for the liquid. Maybe some others have recommendations of other work-hard sip cups?
I would also agree with lllmeg on questioning the lactose intolerance, and good for you for requesting more tests. And for modifying yours and your child's diet - which is very very difficult. Cow's milk is in everything, and my daughter was also extremely sensitive to it. Its in many processed foods - breads, crackers, most restaurant food sauces, butter, boxed add-water dinners, and is disguised under names like lactoglobulin, whey, casin, etc. Also, when there is one sensitivity, there can be many more - very hard to tease it all out. Allergies/intolerances/sensitivities are just hard - and it will require both your vigilance as well as the advanced degrees of doctors to figure it all out. But you are the ultimate and sometimes only advocate for your child - So again I applaud you for requesting more tests, questioning what you don't think makes sense, and trying so hard to make the best decisions for your child.