I wouldn't say he's lost at this point; although it's not looking good with Clinton's current lead. But except for New York, all of Clinton's stronghold states have voted, and Sanders has managed very close races in the swing states, picking up close to half of those delegates. A strong win in California could erase most or all of Clinton's lead; though that state is likely to be as close as the rest of them.
Washington and Pennsylvania are still to vote, and a strong 60/40 win in those, combined with similarly strong wins in the Midwest and Southwest, could erase Clinton's lead and make the superdelegates the tie-breaker vote; which admittedly is likely to go to Clinton, though the past history of superdelegate voting makes that less than assured. Unlike the GOP, the Dem's primaries are all proportional, no winner-take-all, so even a loss in a few states can help if the voting is close enough, and there are no more landslide wins for Clinton as there have been in the South.
At this point, it's looking like Clinton is going to clinch the nomination before the end of the primaries; but Sanders is not out yet, there's still a decent chance he could pull off a popular win, which could sway enough superdelegates to his side to get the nomination. And uphill battle, but not an inherently unwinnable one, as the remaining states aside from New York and California are likely to be strong Sanders supporters. NY and CA will probably be close to evenly split. I'm betting Clinton will take NY and Sanders will take CA; but not by a decisive margin. And CA is one of the last stated to vote. I'm fairly certain that this race will not be decided until June.
By contrast, it's looking very much like Trump is going to get the nomination; while a group of Republican voters are talking about running a third-party candidate if he does. If they do so, that would be excellent news for whoever the Dem candidate happens to be.