Some schools do this to attract their top ranked candidates. Big thread on this for HBS.
If OP's DS really wanted the 2nd choice school, they would have a good chance, otherwise the school would not have contacted them.
Thank you Tiffinboys for this, may I say it was very interesting read. It is possible this may have been the case with our DS. Let's hope so!!
Now its all a waiting process. Second Preference school has said they cannot tell me where my DS is on reserve/waiting list until beginning of April. They have to wait for boys to decline or accept - is this what many parents refer to as First Round?
Many thanks to everyone that has replied! It is so helpful...