There was 1 time I was struggling with my shopping and he helped me.
He came in and I said thanks it was a warm day so I offered him a cold drink
Maybe that led him on
No, that wasn't leading him on. It was offering him a cold drink. It's his problem if he assumes it meant any more than that.
You are allowed to say no. If he doesn't respect that, he's harassing you.
Next time he contacts you tell him, "I am not interested in any sort of relationship or friendship with you. Do not contact me again." Then block his phone number, block him on social media, block him everywhere. If he continues to try to make contact, document everything: take screenshots, everything. And take those screenshots to the police.
Or you could take them round to his house and show his mum, and tell her you'll call the police next time he contacts you. That might work too. But mostly, don't interact with him, don't respond, don't talk to him if you bump into him. Tell him, clearly, you don't want to see him. He might not be willing to listen, but that isn't your problem.