If you want a password to log in, create him a new user account by going to Control Panel - User Accounts.
Set a password, and make it a user account, rather than Administrator or anything else.
Be aware however that this only restricts him logging in, if the other account(s) has/have no passwords he can log in to those easily enough.
Alternatively, set a password on boot, by entering the BIOS (hit delete, F10, F2 or something else whilst the PC is booting, the key to press is usually displayed on screen).
The head over to a section named Security or similar, and set a boot or system password.
Save the changes and exit.
This way, every time the computer is turned on, a password is needed before it will go any further.
To solve the problems of unsuitable activities and sites, I would suggest some form of parental control software, and a stern talking-to.
I'm not too sure what's good and what's not in the software zone right now, hopefully someone else will be able to give you a hand with it.