I don't think the president used his fingers because, in this page ( http://www.mangago.me/read-manga/elektel_delusion/mf/v04/c020/31/), Shunpei thought he succeeded to escape the fingers and, in this one ( http://www.mangago.me/read-manga/elektel_delusion/mf/v04/c021/15/), he said the only interest of Satonaka was to interfere in his relationship with Fumi and giving him fingers.
And later, when Shunpei explained what happened to Fumi, Shunpei said about Satonaka "Then, if he didn't use his fingers there, he wouldn't have seen it (the mole)".
Here: http://www.mangago.me/read-manga/elektel_delusion/mf/c021.5/10/
I think that Satonaka "just" gazed at Shunpei's arse.
You got me... the last thing I'm in the mood to deal with is some pissed off fujoshi over whether or not a fictional character got a finger stuck in his boy-pussy. Believe what ever gets your little fan-girl heart fluttering. It matters little to me. I read and interpreted it differently, that's why I used "IMO" (IN MY OPINION).
So how much did president do to shunpei, I'm guessing it was a finger but nothing else, but maybe I'm wrong and he did it with the vibrator.