They can try to remove him via the 25th Amendment, but if he refuses to accept that he’s unable to continue serving (and you can bet that Jill already has the letter written that Joe is okay), then it goes to Congress where a 2/3 majority is needed in both houses to remove him. In the meantime, Harris is acting president until this is resolved.
Glad someone pointed out that invoking the 25th. Amendment is not an easy process.
Even if the 25th were invoked, Congress could slow walk it right up to the election.