So you have him in judo? They are having them Rendori? I would still stick with my post b/c we have wrestlers come in and use dbls and singles and they get told to use the technique they were shown in class not that wrestling is bad just they want to expand their skill sets.
Make sure he uses his opponents gee to gain leverage. In jujitsu top/bottom position is less important I think but in judo they are looking to pin opponents.
I usually dive for the belly and step over to side control if they don't stop me or swim but there is no one move I can think of that gains an advantage from the start and that's prob why we start from there half the time.
Another thing that sometimes works is getting a high grip on each side of the neck and using body weight to help pull them onto all fours then go for the back. You should push them back first to make them resist and then pull down hard.
Jens Pulver, greatest lightweight striker of all time, no seriously.
"I fear no man, but Jim Harrison and Mike Stone are the two last men that I would wish to meet in an alley fight. Jim Harrison's fighting style is not flamboyant or spectacular, it's just simple and deadly!" - Bruce Lee