Gingivostomatitis can be caused by the herpes virus but can also be caused by other virus'. Just because he has this symptom doesn't mean he definitely has HSV.
I'll second this, when dd1 was tested the only virus identified was chicken pox which she'd had a known exposure to. The cold sores might've been a coincidence.

I'm sorry you're feeling overwhelmed by it all. It's no fun having sick kidlets, and it's been a pretty major run of it thus week. But repeat after me: this will pass. He will be ok.