Make sure that you are letting him know that you are enjoying YOURSELF. If he feels like, for a second, that you are just trying to "get it over with" the mood is shot.
Work that frenulum.
Was just going to give identical answer to above,Hey girl,Ain't you seen American Pie?Maybe Road Trip,not too sure,but one the guys in that movie experiences this.!!
Stimulate his prostate. Works every time :)