Tell your girlfriend to tell him that she is very hungry and to bring more, so next time you're at home you wont need to order a pizza before going out.
Seriously as a man I understand you, you would just like to go to the kid break all his bones and make it clear that he is looking at your territory, its perfectly fine man, thats how it has been from the stone age.
However now we cannot do that anymore due to changes in society and law, most of all now your girlfriend has a say awfully, so in many cases you cannot face problems the straight way but you have to do it the clever one, even if more demanding to your head and your patiente.
So make fun of the kid with her, amplify the fact that you now have a contender and laught about it, tell her that you are envious that you also want some shy kid to buy you presents and her how she did it.
And when you get tired of it, just make sure the kid see you making out and touching her so you mark your territory and make it clear that his sweet princess is not princess but a woman.
