Santa - I don't know the etiquette re being on/off dating sites etc.
sometimes people have told me 'an email came from dating site I logged in to read it' - and it's as simple as that. some people are more curious than others. If I'm dating for example and don't want to close internet dating then I'll see eg Match's emails in inbox but not open them, eg try not to log into the sites. others may not be so careful.
I'd be more wary in your case with the fact he's booked the holiday, not asked for any money from you (does he expect any?) and also knows backstory re ex ripping you off.
I wouldn't go on holiday properly (discounting short breaks) with someone until I'd known them at least 6 months.