How to tell boyfriend he went too far?

My boyfriend of two months and I had a pretty heated make-out session tonight. Nothing happened that I was uncomfortable with... However we pretty much reached the limits of how far I want to go and it hasn't even been 3 months! My boyfriend is super understanding and I know he would never pressure me into doing things I don't want to do. However, I feel like I still need to convey my feelings to him. He's my first boyfriend, so everything we do is new territory for me. On the other hand, he's had two relationships before me and I don't know what he is accustom to. I just really feel like we should take things slower, but I don't know how to start? What are some good things to say that will help him understand my thoughts?