he will have been napping at this moment.  此句用的是一个完成时，但是，后面却用了一个表时间点的时间状语at this moment, 这显然是错误的。 完成时的标志是by, 可以接以下时间状语，带by的，within的，during的。  before, in the past, in that period, in that time, in the past 10 days/years/months, by now/that time, these days, recently, in recent days/years/months， today, this year, this month, this week 等等。 

你可以说 he will have been napping by this moment. 
而he will be napping at this moment.就没问题，一般将来时接时间点，正确的表达，表示我在此时打盹。