How to prevent a child using an adult account on PS4?

Solution 1:

According to the official PS4 user manual:

Block Other Family Members from Logging in as You

To prevent children from logging in to the PS4™ system using a parent or guardian’s account, set a login passcode. Log in to the account you want to set a login passcode for, and then select (Settings) > [Login Settings] > [Login Passcode Management].

How-To Geek also provides a step-by-step guide with screenshots.

When you're stepping away from the PS4, remember to sign out, put your PS4 in Rest Mode, or turn it off entirely to prevent your kid from accessing your account while it's still unlocked and you're away.

And don't underestimate your child either. If your son really wants to use your account, he could observe you as you enter your passcode. When properly motivated, children are pretty good at circumventing security.