imessages not working

Hi,

My son is having problems using iMessages on his ipod touch. Actually, it was working fine until recently. He activated it using Apple ID and validated it. He has been sending and receiving messages without a drama until the past couple of days. All of a sudden he can no longer send or receive them. When you click on the iMessage icon, it comes up with the "get started" screen and asks for his password. We put in the password (which is correct), it goes to the next screen where the correct email address appears and then it returns to the previous Apple ID screen. It doesn't give any validation or other error messages. It does the same thing if we try to access it fromt he settings menu. Anyone know how to fix this???