[Pierrot]

Others have already said this, but I still wanted to say you're not overreacting. Tell him bluntly that you're not interested. Don't offer excuses, like that you already have a boyfriend or that you work a lot, etc. Just say it straight up, and keep saying it. There's not even necessarily a need to put it kindly, or politely. And once you've established that personal boundary, keep it that way. The urge to be seen as friendly and well-mannered is conditioned into most people, but in this case it's okay to maintain a distance. Please take care.