I don't know much about Guppies (just got my first ones today acually), but here are my thoughts, looks to me like over feeding. From the looks of the fish swimming behind him/her they are slightly large in the belly (yes I know what a pregnant fish looks like
). How much do you feed, could your son be sneeking the fish some food (could hear him in the background of the video)? You may want to post this in the general freshwater section, doesn't seem like people go into the disease area much.