IMO, 9 is old enough to understand why something like that would be an inappropriate action, and your best bet is just to talk to him about it as frankly as you can manage!
the major thing i'd talk about is doing things to others without their consent. that is the biggest issue here, i'd think, considering that plenty of people have memories of doing things like that with their peers or siblings. it's all within the range of normal, but if he's not able or willing to tell when the person he's with is uncomfortable with an action, that could definitely open the door to less normal, healthy actions.
have you talked to him at all about sex, sexual development etc? 9 is such a tricky age... many kids that age are having powerful urges and desires that are on the edge of sexuality, but they are still acting on them in very innocent ways.
anyway, the most important thing to discuss would be that what he did was wrong, not because of what it was or who it was with, but because the person he did it to was not happy with what he did. if someone didn't like being kissed by him, it wouldn't matter whether it was a 4 year old, a 9 year old, or a 90 year old that he kissed.
it's a delicate situation! i think you'll do fine, especially based on your reaction to your younger child!