I don't have an answer for you since I don't use the usage controls, but when your overall usage hits 2GB, I assume he has consumed the majority of that data. At what point in the month does your combined usage hit this level? 1wk, 2wk, 3wk? Another question, when your son is cut off I assume you and your wife still have access to the data?

Seeing as you have allocated your son 2GB and your allowance is 4GB, I assume you and your wife each use less than 1GB/month on average. I have come up with the following suggestion you can try until you can gauge how well your son has his data usage under control.

First of all, if it is available, have all of your phones set to automatically connect to wifi while in your home. No need to use cellular data if you have free wifi available.

If your combined usage hits 2GB after 1wk, then on average your son will have used 1.5GB of that amount with 0.5GB coming from you and your wife. In this case, you can increase your son's allocation to 2.5GB and that will give you and your wife 0.75GB each to use for the remainder of the month which would fall in line with your 1GB usage/month.

If your combined usage hits 2GB after 2wk, then on average your son will have used 1GB of that amount with 1GB coming from you and your wife. In this case, you can increase your son's allocation to 3GB and that will give you and your wife 0.5GB each to use for the remainder of the month which would also fall in line with your 1GB usage/month.

If your combined usage hits 2GB after 3wk, then on average your son will have used 0.5GB of that amount with 1.5GB coming from you and your wife. In this case, you can increase your son's allocation to 3.5GB and that will give you and your wife 0.25GB each to use for the remainder of the month which would also fall in line with your 1GB uasge/month.

Hope this helps until your son can get a grip on his data usage habits.