is this anxiety? in child...

Hi....my son is 11 yrs old. I am wondering if he has some sort of social anxiety.
At home, he tends to be loud, bossy. At school, he is very quiet. Yesterday, we were at a fair and the kids were having fun...playing games, going on the rides etc. My 11 yr old son was drinking a "slush" when a group of girls from school spotted him and all came running over to him , yelling "hi steven!". Instead of
being happy or just saying hi back...he did something very bizarre. He held up his drink and threatened to throw it...then some did splash out and splash one of the girls. I was sort of in shock...like what are you doing??? the girls looked almost scared, like wow, hes a weirdo...type of look. I was upset with him but felt so bad for him at the same time. After that happened he sort of got away while I apologized that steven got them wet! I yelled at him for acting this way and then he followed me around with his arms folded in front of his chest looking like a nervous wreck for the next hour or so. Then we bumped into the girls again and I asked him to apologize and he did (which was big for him) and then I let him play some games again....which he got very emotional because he couldn't win anything and his little brother did. oh boy! I just don't know what to do for him to help him....any advice appreciated. thanks... Tweet