Non drug-induced hallucinations

I know a fever can make you hallucinate, so he could snog someone with swine flu?

trauma-induced craze? perhaps blunt force to the head? regression to a previously repressed state?

I assume you're talking about visual and auditory hallucinations, like people and other things appearing and making noise?
There are many medical conditions that can causes hallucinations, for immediate, your character could have a seizure. Not like a huge seizure, necessarily. But hallucinations do happen after seizures.
Your character could have a psychiatric condition begin to emerge, such as schizophrenia.
For immediate, that's as much as I can come up with that don't involve injury.
If the character's injured, they could have head trauma, or be hallucinating due to shock or loss of blood.

There are times we're hungry, broke, and FREEZING, and I wonder why the hell I'm still living here. And then they call. And I remember.
I love Rent. Most epic win movie/musical EVER.

Prolonged sleep deprivation could do it. Exposure to some chemicals. Possibly not eating for a while and/or eating only weird stuff so that you develop a serious electrolyte imbalance.

I fell into sleep-deprivation hallucinations once on a long road trip, that was a most interesting trip to Disneyland. I think it was something like 3 days with no sleep at all.

Just wanted to say that schizophrenia-related psychosis isn't a sudden thing. Your character would experience quite a few symptoms before they started to hallucinate.