Christian and Alan have become increasingly close over the course of the shoot. With Christian's family being so far away, and not able to visit very often, Alan has become a father figure of sorts.
When his young protégé comes to him, out of everybody, babbling about his confusion, and his attraction to Sean, Alan doesn't quite know what to do. Christian obviously needs advice, and the best way to start would be helping him work out if he really is gay or not.
But the boy's so young, and Alan is reluctant to take the obvious action. Finally, he decides to just bite the bullet. It probably won't do any harm, after all.
"Christian. Come over here. If you don't like this, just stop me, OK?"
And he kisses him.