Oh HW, my heart sank when I read your message.
In my experience, they will lie and lie unless you saw him do a line he won’t admit it. There has to be another reason yada yada.
The tests were negative, then he goes away, your gut tells you something (which is probably right) to want to test him, and he fails… no surprise there. We always JUST know.
I have no advice really other than leave it a week and test him again, you’ll know then whether it’s a blip or it’s started again.
Sending love x