Can you get a child christened using a different name from the one that is on the birth certificate?

You can have him baptised with any name you like - provided it's acceptable to whoever is carrying out the baptism. He can be known by any name you care to give him - or later, by any name he chooses to use. The law on changing names officially vary between countries and states, but once he gets to 18, he can change it quite easily without permission from you or his father.