(1)everyone,everybody,someone,somebody,anyone,nobody等,附加疑问句中主语一般用they（口头语,非正式文体）/he（正式文体）
(2)当陈述部分有never,seldom,hardly,few,little,barely,scarcely,nothing,none,rarely ,no,not,no one,nobody,neither等否定意义的词汇时,后面的反意疑问句则为肯定形式
所以这句应该改为
Nobody believe that he can fly,do they?