2000 sexual partners is an awful lot IMO.
I simply can't get my head around the logistics of someone who has had sex with that many people. I am struggling to fathom how anyone could manage it. Or why anyone would want that many sexual partners. 
And regarding the comment '2000 sexual partners is pretty standard these days;' I am no expert and I don't know everyone else's sexual habits, but I doubt very much that this is true. I bet the vast majority of people have not had sex with even a tenth of that amount.
I remember a similar tale a while back, (in the news,) and someone commented that shagging one partner after another, night after night, (til you are into the multiple 100's,) is really no different to masturbating.
Rightly or wrongly, I wouldn't want to be with someone who had had sex with 2000 people. I am not saying they are awful or nasty or anything like that, as each to their own, but I just couldn't be with someone who had had sex with that many different people. As I said, I really can't get my head round it, at all...... As a few posters have said, it does sound like an exaggerated amount. Can he prove it? Probably not.