S=(1+x)1000+x(1+x)999+x2(1+x)998+⋯+x1000
This is a geometric series with first term (1+x)1000, common ratio 1+xx, and 1001 terms.
S=(1+x)1000⋅1−1+xx1−(1+xx)1001
=(1+x)1001−x1001
Required sum = coefficient of x499 + coefficient of x500 in (1+x)1001−x1001
=1001C499+1001C500=1002C500