Maybe slightly OT, but I have NEVER believed in multiple citizenships. Under no circumstances.
If born in US, then you're a US citizen (now maybe that should be revisited), his father was presumably here on a temporary work assignment. Seems silly that he got hassle from US Immigration tho'.
I'm not sure what the deal for children of foreign parents is in UK as they've diddled around with it so much but if you're a British citizen you don't lose it by becoming a citizen of another country.
Mr Boris is in fact a Tory Member of Parliament.