There are a few different ways of doing it.... If you click on his name a "Membercard" pop-up will appear. From there you can see quick stats about his account and also a link to "Start a new conversation". That is a *private* conversation that nobody else will see unless you invite them to the conversation. If you click on "Profile Page" instead you will see his public profile page. From there you can write a "Profile Post" (sort of like writing a message on a Facebook "Wall") that is public that others can see & comment on.