@DaPou - where are you seeing this usage of 78MB? If it is his phone, I would suggest you do not go by that. You would need to have the phone set to the start date of the plan cycle, and some things could skew the results.
It is best to go by the My Account after refreshing with the circle with the arrow button (as mentioned above).
Also, to see full details of the data usage choose to DOWNLOAD the usage from the start date of the plan, as this as the accurate results compared to the overview section.
Note data gets logged normally twice a day, so you will won't see live usage right away.
My son has a data plan for MP per month. According to the usage, he only used about 78MB, however according to the general overview, there is no MB left.
Could anybody explain how this is possible?
Don't worry about the usage, just use it until the data use up, then your son can't use data.
Before logging into self serve:
Also, try tapping the highlighted area when logged into self-serve for a refreshed page version: