:D
Yeesss - I can imagine you're not amused. But actually, I suspect it really is just his teenage years. What he's doing is trying to get a response from you, probably to entice you to play. Any response is better than none though. Most likely, the first time he did it was just fluke placement really, he might just have easily tried biting your feet, or jumping on your back. But I bet he got a reaction

Not just from you either - if your girlfriend thinks it's funny, then it's probably double the fun and attention for your pup.
Well, the best way to stop him is to spoil his fun. That means either zero reaction when he does it (no reaction from anyone, so your girlfriend can't laugh at him either). If that's not possible, or he doesn't quit quickly, then when he does it, get up and leave the room (shut the door so he can't follow). Exclusion is a pretty big punishment for a dog, and it is absolutely the opposite of what he is trying to achieve - so that is a fast way of extinguishing the behaviour. Getting upset at him won't really help - because that is still a reaction and he is still a teenage brat.
Best of luck
