He absolutely can be replaced at this stage, and by nearly anyone.
Only if they can convince him to step aside and let someone else run. At this point the voters have selected 3,904 delegates who are contractually obligated to cast a vote for him at the Convention. If the delegates somehow simply ignored the primaries, they’d be quite literally ignoring the will of their voters and taking matters into their own hands. It’s alarming how many on the left (who presumably had a problem with the DNC’s treatment of Bernie in 2016) are cheering for the DNC to heavily influence the primary process again. I don’t necessarily disagree that something drastic needs to be negotiated, but the irony of this is really hard to ignore.