Oooooooh this is a toughy.
On one hand he creeps you out but at least he is being nice about it. in a strange sort of way
If you suddenly avoid him and get managers to remove him from the store etc you could push him to do even more extreme things.
He sounds like a complete nutter. Can i ask how old he looks and how old are you?
I would definately say to him the next time you see him that your boyfriend was upset about the card, and in turn has made you upset.
Its important you tell your manager, i'd go to the highest manager (store manager etc) and tell him / her about it and how uncomfortable you feel. If they have any sense (not many managers do
) they will make sure you get into your car safely and the end of every shift.
I work in Tesco
part-time and a few years ago a young girl (16 years old) who worked there was followed by a man when she was walking to work. He then followed her about the store. She immediately told the boss and there was police at the store quicker than you could say... Tesco's way better than Asda.
Anyway i hope everything turns out ok in the end. Maybe he's just lonely, but he sounds like a complete nutter.
paul
edit:
just re-read the card and the more i read it, the cleverer (sp?) this guys sounds. Very cryptic but its all based around photosynthesis, he's comparing his relationship with you to that of a plant requiring photosynthesis to live and surive.
i.e being dependant on someone or something, benefiting by being with someone, interacting with someone, only seeing them during the day, photosynthesis occurs during the day, etc.