Speaking as one who lived through two decades of hedonism, and racked up a pretty impressive / scary (delete as applicable) number, I can assure you, he's lying about the 900.
It might indeed be a lot, a very very large lot indeed. But unless one keeps a meticulous diary, one tends to forget, plus chuck in a couple of long term relationships, and inevitably you'l see some partners for a few weeks, rather than just a one off. Unless he's been regularly participating in quite large orgies, I can't see this. Or maybe he's rich / famous and has virtually unlimited offers?
As for the 'get him STI tested' brigade, hmm, do you honestly think if he got something 3, 7, or 25 years ago, he might not have been made aware of it by now?
And out of interest, during my twenty plus years of largely uninhibited sexual play I never once caught anything and only occasionally used condoms.
So would I be bothered by having a partner with a large number? Not at all. My decade plus partner is no shrinking violet, but I have no idea of their number, nor do they know mine - why would one ask?