why does Family Share allow you to log in only twice a year

Do you mean that his device is disabled due to too many Lock Screen password attempts? If so, then follow the instructions in this support article that best apply to your situation:

iOS: Device Disabled

If that is what happened, then it has nothing to do with Family Sharing. It is a normal Device Disabled situation that would happen any time someone put in the wrong Lock Screen password too many times.

Cheers,

GB

Interesting...Thanks for posting the screenshot - I was not aware of that message/limitation. I would suggest that you contact Apple Support about this. They probably can get you straightened out this time. They may also be able to tell you if there is a way to prevent your son from being able to "Stop Family Sharing". Here is a link:

ACCOUNT SECURITY CONTACT

Best of luck!

GB