He's just fooling you
You should ask him how to calculate this kind of question.
Because 1 * 2 * 3 * * * 999 = 999!
! It was factorial, and the result was abnormal.
1! = 1
2! =2
3! =6
4! =24
5! = 120
6! =720
7! =5040
8! =43200
The latter figure is irregular.