I am sorry you have to deal with this.
Here are a few things you need to take to ensure you feel safe and secure in and around your office building.
Talk to your boss and the HR manager. Let them know about everything that is happening to you. The HR manager will be trained in handling situations such as a ‘stalker at work,’ and they’ll know that your workplace is legally obligated to making you feel safe.
If I were in your place I would ensure that I don’t find myself alone with this guy. Can you change your working hours to a time when this stalker won’t be around? This guy sounds creepy and until the matter is resolved, you should change your work timings. Probably, start your shift at 9 a.m. when there are more people around.