if you have a passport made for a 6 month old, and when he's 2 he looks quite different, would this cause problems at the airport, he's got more hair now, just asking for a friend.
My daughters son had changed the same amount, but the check in girls loved the 6 month old picture of him and were passing it around showing each other. It did not cause any problem.
they will go by the general face shape and skin tone/colour and eye shape etc - certainly never had a problem with mine and my youngest first passport picture was about that age and he had very dark straight hair and by toddler age he had a pile of blond curls :o)