I don't know exactly what you mean by "safest". Does that mean not getting any malware on your system or games suitable for children? When I used to be into that type of stuff, I always used http://www.shockwave.com/
. It has a wide variety of high quality online games. All you need is Shockwave Flash and Shockwave Player. You probably have both of these on your system if he has played games online before. For almost all the games you don't have to install any software. If the site asks you to load anything with "WildTangent", say no. It is spyware and will invade your privacy, so please avoid at all costs. Other fun game sites are http://www.candystand.com/
. Just be sure to avoid the "Hyperload" (junkware) option as his games will already be loaded into your cache after the first time laoding the game. You cache is online content downloaded to your computer, ready to automatically start the next time.