If you'll be using a different account, all purchased games will be lost. You can transfer save data to a computer, use the new account and download/setup/install everything again, then copy the game saves back tot he vita, because when you switch accounts, the memory card will be wiped. Purchases are tied to a PSN ID, not person, so if it's a different ID even if it's the same person, the game will be lost.
The only way to change the PSN account used by a Vita is to restore the Vita to factory-state using the "Restore the PS Vita System" command in the Format section of the Settings app. This will erase everything on the console. All game save data on the PS Vita is tied to the PSN account configured on the Vita. The save data cannot be transferred to a Vita that uses a different PSN account, so your son will lose all of his save data if he changes the PSN account on the Vita. There is no way around that. If he purchased and downloaded games using the first PSN account, he will not be able to install or play any of those games if he changes the PSN account on the Vita. The games can't be transferred to a different PSN account, and the Vita can only play downloaded games purchased by the PSN account that it is currently configured to use.
Just click the "forgotten password " on the sign in screen from a computer and they will send you a temporary password to log back into the account.
This only works if you know your email account and password. But same thing could apply there as well ... there are always forgotten password thingys to click and reclaim things you think are lost.