Need help with likely child sexual abuse

I will try to keep it simple here. Someone messaged me online, asking if i wanted to buy pictures of his little sister, implied explicit. I continued the conversation until i got his discord, then ended the conversation. I stalked his accounts and i know where in general this person lives, as well as a likely highschool. There is some more information available too.

I need help in, i dont really know what to do. I dont live in the USA, where this is taking place, i was only able to screenshot the second half of our interaction, where only "pictures" and "risk to myself" (the user) are mentioned. I feel this is too weak to contact authorities, but i need to atleast contact someone close to this guy, to keep his sister safe.. I wanted to post this on the region specific subreddit, but i know the user has a reddit account and didnt want to risk it. Thank you for any help.