It was naive, he never imagined the British public to be quite so fecking idiotically stupid/a bunch of racist cnuts.
Think that analysis oversimplifies things. Definitely a huge group in 'Middle England' and beyond who followed the jingoistic/bigoted Boris Johnson/Nigel Farage reasons for leaving, but a lot of people voted for it simply because it guaranteed change. That's the real key behind why people you wouldn't normally expect to back right-wing political movements went along with Brexit.
Take where I'm from (and still live), in County Durham which, aside from the university, has been declining for the last 50 years or so. We lost thousands of jobs from the 60s to the 90s as mining and industry was wound-down, with no investment coming in to replace the jobs and the wider communities those jobs underpinned. Up until 1997 people thought 'things will change when the Tories are out'. Then we had 13 years of Labour, with Blair himself having his seat here, and nothing really changed. Stuff like SureStart and investment in the NHS took some of the sting out of it but really we needed an investment and infrastructure strategy and one wasn't forthcoming. God knows the New Labour era until the crash was like the land of milk and honey compared to 2008 - 2018, but you can see why people who aren't necessarily politically engaged and who live in areas where things don't get substantially better regardless of who is in power are easily convinced that their problems lie elsewhere, or might be tempted to return a result that would be a slap in the face to politics in general. Not that I agree, for what it's worth I voted Remain.
Remainer politicians of all stripes' message in my area, and post-industrial areas in general, should have been to point out that whilst the UK
as a whole gives less money to the EU than it gets out, the EU pumps huge amounts of funding into the poorer parts of the UK that they wouldn't otherwise get. That argument was never made as it would highlight the failure of both parties to address regional inequality. Arguments about the larger economic benefits of EU trade, or the benefits of EU migration were never going to work in areas which hadn't tangibly benefited from those things.