You need to show him that older wiser people who have spent a lot of time in London don't agree with buying straightaway.
Being personally responsible for a year of rent - let's say 12-18k - is a big first step, and not to be dismissed lightly.
Being responsible for a 200k flat is a whole different thing. Anyone who lived through one of the last 2 downturns in housing prices will tell you the worst was the small flats.
What if he pays 200, and then doesn't make it through the job probation?
What if he loathes London with a passion?
What if his company gets bought out/downsized/merged and he's out of a job?
What if he pays 200 and then finds everyone he gets on with lives in a completely different part of London?
What if he pays 200 and then in 2 years it's worth 160?
What if he pays 200 and then he finds he's in the middle of a house upgrade/the block needs attention, and he's on the hook for 2000/5000/7000 contribution to a new roof or repainting the entire block?
All these things happen everyday.
I grew up in London, I saw my parents buy there, I rented 7 times in 10 years before buying my own house in the arse end of nowhere, so I'm not talking out my ears. I made a fortune from mine, but only by being savvy.
We're not being killjoys, we're being realistic.