My son's USA passport expires in June 2016. We are traveling to Italy in April 2016.
Does he need to get it renewed before we leave?
I'm not sure if there's a specific rule on traveling with an 'about to expire' passport.

Never mind - he needs to renew it per Passport Agency regulations.

Thanks!

It needs to be renewed if there are less than 90 days left. He could technically stay 90 days in a country but would be there illegally after his passport expires.

Some countries require 6 valid months on your passport. So, yes, get it renewed.

Can I still book the flight and enter his passport credentials when I book it even though he's renewing it, thus the expiration date will change?

I am glad you got your answer as this happened to my sister at the end of November. I booked her ticket back in May/June. When putting in her passport information, I noticed the expiry was end of Feb 2016. I booked the ticket without putting in her passport info and said to her, they might not let you go due to the expiry date. I suggested she renew. Well she should have listened. They let her fly to London in Oct but probably should have stopped her. She tried going with me to Germany in November and it was a no go. I doubt lesson learned but glad you looked into it and renewing before he goes.

Not only will the expiration date change, so will the passport number. You can still book your tickets now, just go back in and update the p'port info after the new one arrives.

It says passport must be valid for 3 months after date of departure .

Good to know, thanks to all.