The kid I used to know has grown into a charming and attractive young man who looks even older than his age.
To that end he may try out various behaviors on you, flirtatious behaviors that have gotten a certain reaction from girls his own age.
You are a source of support and comfort.
To that end, I do not think it is a good idea for you to try to discuss with your stepson any possible sexual attraction between you.
It may be that he is learning to use his seductiveness as persuasion and power.
I know I'll never make a move on him.