"Sean" (properly, Seán) is actually an Irish Gaelic name. Irish, like English, features very many non-phonetic spellings. If the guy deliberately made you feel embarrassed for mispronouncing his name, he was just being a dick.
'S' for example is always pronounced like the English 'sh' if it's next to 'e' or 'i', and like a regular English 's' if it's next to 'a', 'o' or 'u'.
There was an Eoghan, though.