Just tell him the only possibility of bricking is during the upgrade/flashing process, and it's only if you do things wrong. There is no chance of bricking once you are actually running the custom firmware (unless ofcourse you start using programs that access the flash, but that's what recovery mode is for).
You should give him some links to 3.03 OE-C information so he can see why people use it :p Even if you don't plan on using ISOs or homebrew 333mhz will speed up some of your games and make them play smoother.
[additional]
Just checked your profile and you're 17!! and your Dad won't let you hack your PSP? My little brother is 14 and my Dad and I both trust him not to go messing with the flash and stuff. He even helps me debug my games and apps. Sounds like your Dad really has no idea about the PSP and should stay away from it, there's probably more chance that he'd be the one to brick it.. lol