Tell him yes, of course he can come flying.
Suggest to him to take this sick bag, as you present him with an empty 25KG brown paper potato sack, tell him to take 2 as they fill up easily.
Take an odd look up at the clouds and mutter to yourself....f**k this is going to be rough, and say "hey mate" better go and use the bog before you get in..." have I mentioned your seat belt doesn't work and I only have one parachute, my last passenger has yet to be found with my last one... and you ain't bloody getting mine.
This usually assures them.