Block. If it was a close friend I might give different advice, but this is a stranger on the internet being annoying (or worse). Responding to him will only make it worse. Block and move on. Note: you (both) might still think about it later and you might feel a pang of guilt, or shame, or worry, or whatever. You shouldn't, because you're doing nothing wrong, but that's fine if you do. It happens to everyone. You'll think about it less and less over time.
Also, I would give the same advice to someone who isn't a minor, and you're right to question his actions. He shouldn't be asking/nagging for free work. No means no and is absolutely the right answer.
If he knows your girlfriend is a minor, that just moves this to another level and she shouldn't even hesitate to block him. If he gets around the block report him right away.
Well, just their juice.