heres the problem.

we currently an imac with itunes/music on an external drive which we both sync our iphones to, but my son is getting a new mac which will be in his room
(but will be set up on the same wireless network)

-my son will want to start syncing his phone using his new mac
-the existing music is on an external drive that is connected to the original mac
-there is far to much to copy to his computer unless i get another drive (last resort)

also when he first syncs his phone with his new mac will it have to be wiped out

Set the iPhone synchronization to "Manually manage music and videos"...