website won't connect to server under 1 account, but does under mine

I have my son set up with parental controls, and I am administrator. I need him to go to the website xtramath.org, and have that set up to allow him to go to it. But under his login, I get an error and it says it won't connect to the xtramath.org server. Under my login, it goes there fine. How can I resolve this so he c

Thanks for any help.