Singh middle name is in use by Hindus from ancient times
by Hindu Rajpoots or Thakurs.
However Sikhs also use Singh. All Singhs are not Sikhs
as the common perception is. Any North Indian knows this.
the word singh varies. Sikhism came into existence in 1699 but even before that people had singh name. for example akshay kumar, that guys name could have been akshay singh. and anyway its one person wish what he wants to name himself.