It seems rather harsh to be going for sexual harrasment or assault. Yes, I would not suggest being alone in his room again and if you don't want to, don't talk to him. However, grinding is pretty commonplace and he might not have known you were not comfertable with it. Saying he touched you 'down there' isn't exactly very descriptive. That could mean anything from he put his hands on your hips to stuck his hands down your panties and fingered you (and if that is the case, he did assault you)! Just make it clear that you were not ok with what he did, and if it was egegreious I'm sure you can figure out who to contact.