Your son may keep Ad-Aware on his system, but must disable the real time dectection component Ad-Watch. I have used Ad-Aware for about six years now. Ad-Aware can detect threats that NIS misses, I have used Ad-Aware to clean up clients systems' when even MalwareBytes and SuperAntiSpyware miss the threat. Tests are currently being conducted on Ad-Aware 2010 to see whether or not its Real-Time protection interferes with other security software's Real-Time protection; it is Lavasoft's goal to prevent conflicts of this nature.
In a nutshell: either remove or deactivate Ad-Aware It depends on whether your son thinks Ad-Aware is worth keeping or not, but its realtime protection has to be disabled in order to avoid conflicts with NIS 2010.