I seem to recall some folks mentioning a September 25th “drop-dead” date, but I’m not certain of that.
If a candidate does not drop out of the race by 5 pm on the 6th Tuesday before the election then his name remains on the ballot regardless. That is the September 25th deadline you referred to. If he were to drop out after that and if enough people voted for him to win the election then the seat would be declared vacant. The only exception would be if a candidate died before the third Tuesday before the election. Then his/her name could be replaced.