The IgM is an incredibly unreliable test, and they shouldn't even be done on him because he's not a newborn, and it wouldn't be a new infection, which is what the test looks for. The IgM antibodies only last a few weeks.
Here are some things you can read that may help:
You should ignore his IgM tests entirely, and pay attention to the IgGs.
His tests don't matter as much as yours, though. His do help in unraveling your mystery, but your tests are more important. You've had a bunch of negative tests, including the WB, all done at the appropriate times. Even if he does have herpes, you don't.
I'm sorry you still don't have an answer, though. Are they investigating Behcet's?
Here's some info on diagnosing it - https://www.mayoclinic.org/diseases-conditions/behcets-disease/diagnosis-treatment/drc-20351331
That could just be pimples. Or an allergic reaction, or a long and varied list of other things. Sometimes, a pimple is just a pimple.
How long is the WB supposed to take?