When my son drifted into Autism

I  have  no  idea,  when  my  son  started  drifting  into  autism. All  I  know  is  he  stopped  and  lost many  things  that  he  used  to  do  as a  baby  and  toddler.  He  had  good  eye  contact,  he  used  to smile  and  ask  people  to  carry  him  gesturing  with  his  hands.  He  used  to  say  and  wave  bye  when he  was  a  baby.  Later   he   stopped   giving   eye   contact.  He  even  started  to  say  a  few  words  but later  forgot  or  lost  his  speech.  We  started  noticing  some  unusual  behavior  around  3  and  4 years of  age. It  started  by  scribbling,  he  used  to  draw  circles  over  and  over  non top.Whenever  he  gets  hold  of a book, when one page finishes he turns to next page and like that he used to finish one whole book at one  go.  After  that  he  used  to  turn  pages  of  books  and  fold  the  corners  of  each  page  and  then he used to do something very unusual, he used to fill and block people’s faces in pictures. Slowly he lost interest  in  books  and  pencils.  Then he  turned  to  hand  flapping  and  still  does  it.   He  has  regained  his eye  contact  but  still  he  is  non  verbal  and  still  it’s  difficult  for him  to  hold  a  pen  or  pencil.