Imho you should say something. It's been my experience that people love to have the opportunity to talk with people who knew and remember their loved one. This is especially true if it's been a long time since they passed. I mean your coworker will probably have friends/associates /colleagues/neighbours etc who came in to his life after his son passed and therefore he can't even reminisce with them about his son. You have a bond with him he won't find with just anyone.
Yes, he may get upset....that's natural. But I'm sure having someone that shares precious memories of his son with him will mean the world. Just let him 'take the lead' (lol) in when and how much he'd like to open up to you.