I found marijuana in my son’s room?

My son is 13 years old and started his first year in a new school this year. It’s his first year in middle school as well as his first in public school so I was a little worried that my son might encounter some bad influences. He’s been a good student and even better kid his whole life. I understand it is not as dangerous as other drugs but it is a gateway and illegal. How should I approach this?