A Kitten's eyes usually start changing from a blue colour at around 6 weeks of age but looking at what his eyes look like in the photo I would of said a maximum of 8-9 weeks at the time of the photo.
It could be that he has Norwegian Forest or Maine Coon in him which is making him seem to be so big, plus he is now getting regular food and good care which will help a kitten flourish and they do seem to grow quickly if you imagine that when kittens are born they will actually fit in to a females hand.