Depending how you got the change done it should be changed instantly. It's possible to request the change to reflect on the next bill cycle, but unless you specified that it should be changed right away. However, if the change is applied in the middle of the bill cycle, you will only have half of the data allowance.
To my knowledge, all changes made through self serve go through immediately. Customer service reps do have an option to make the change at the start of the next billing cycle to avoid proration charges. Why does your son say it isn't effective until next month? Is data simply not working? Have you checked if there is a data block add on on the account?