Hey Carley,
While the boy is a step-son, the fact the “kid” is doing it in his house is a much larger factor IMO.
It is never late to make a rule especially if it is an issue that is bothering the guy. Its his house, period.
BTW, I am a 47 year old male engaged to a similar aged gal. When we visit both sets of our parents, we sleep in separate beds. Neither of us would dream of putting our parents in that situation.
You have respect for your parents, evidently this boy and his girlfriend do not.
Good for you.