I viewed the longer video and saw no obscene gesture. Nathan Phillips is an agent provocateur who's appears at a number of demonstrations, many times playing the part of the aggrieved.
Besides the second act at a fraternity, here's he is at a Trump hotel in 2017.
And, an earlier picture at a oil pipeline
FTR. my wife agrees with you. She watched the long video and said the teen's gesture wasn't obscene, just rude. So I'll retract the obscene part. In case any don't know what the hand action meant he was calling Phillips a drunk. Whether that was obscene or not, he should have just ignored the old man and not come across as a spoiled brat. And with all the cameras videoing, why didn't he expect it to be on the Internet?