In that case you have to buy a second licence key. If you buy a LSA key (5 clients minimum) you can add more clients using the same key.
You don't need to have the same key to use FM in a network, but it is convenient especially installing larger groups.
Hope this helps,
Ruben van den Boogaard
The answer depends on what your needs are. If you have a single licence you can install FM on two machines (I have it on my desktop machine as well as my laptop, for example), but you can only run one of them at a time on the same network. On the other hand, if both you and your son will be using FM regularly in the same network you would be better off with separate licences.