The best thing to do is ask your son for the password. If he cannot remember it, then, you can try to logging in as the administrator. That is unless the admin account is the only account on the laptop.
If your son does not remember the password, then you will have no other choice than to restore it to factory settings. However, this means that any important documents on the C: drive will be deleted.
It is always a good idea to put a password on the laptop even if you are the only individual using it. If you want to allow your son to use the laptop to play games with in the future, then you should create a standard account for him. Standard accounts do not allow users to install / uninstall programs. Only the administrator can install / uninstall programs this prevents the laptop from being modified by the user whether innocent, a prank or malicious. From a parental point of view, you would have complete control over what games he can play on the laptop since you will be the one who will need to install it for him.