It depends a lot on what is the seriousness of the suspicious activity. The best you can do is lock certain websites so that your child can’t visit them and restrict the programs that can be run on the laptop. But nothing can be full proof , a smart child can find a way to bypass the restrictions. You need to talk with the child , not while he is on the laptop , but maybe at some other time. Don’t scold him or try to advise him, children really hate pep talks and to be treated like kids. Also try to get him interested and keep him busy in activities or games outside the computer .
It’s easier said than done . It takes a lot of patience and time . You need to be calm at all times and never lose your temper , never be angry or agitated.
I’m sure this is a passing phase.