הכרויות סקס חינם|דייט

I 23F was having a rough discussion with my bf 23M of 3 years today. He was washing dishes, and I wanted to take another cup to the sink. It turned out it wasn't empty, and a few drops of water fell on his bare leg (he was wearing shorts). He yelled that I'd "spilled water" on him and that I should wipe it off. I rolled my eyes and turned to grab a piece of paper, telling him to calm down, it was only a few drops of water. Then he grabbed the cup from the sink and poured all the water on my pants. I find this behavior so childish it's pathetic. When he gets angry, he often does something to spite me. How should I approach this?