You're probably correct. If the student isn't contributing, that means that he's most likely not even logging into the class, which means that he can't read postings if he's not logged in.

Perhaps the best way to handle this is to send the student a private email or call the student if you have his phone number. Maybe there's a legitimate reason why he hasn't logged in. Also, he will appreciate not being embarrassed in front of the rest of the group.