What action to take when questions regarding a published paper are ignored by its author?
How do you know he ignored the email?

Maybe he never received it because it was filtered before he had a chance to see it. Maybe he hasn't read his email this month because he's on vacation. Maybe that email address was good when the paper is published but not good now, but it's also not bouncing. 

If one channel of communication doesn't work, try a different one. Call his office phone or send a letter to him. Write to a different author saying you've been trying to reach the corresponding author without success and you want to check the email address. Don't assume anything when you have no information.