Hello amag1975
It is vacation season now, and it was a wise decision to reserve seats on the IC2 trains.
Are you sure it is a double deck train? Can you tell me the train number, please?
With double deck (duplex) trains, seats 0-99 are at entrance level, seats 100+ are at the upper deck, just as in a hotel where the 100 stands for 1st floor. There are signs on the platforms as well.
You can still change seats to the upper floor if there are enough seats left and in case your son never rode on a double deck train before.
Kind regards,
Roland