You can use the chat... or remember his name and send a message over PSN Messanger... if this is blocked, you have to send a friends request. Works on PS4 Messenger too. Available for iOS and Android.
I have found that to be the easiest way to have their info on hand if I want to message them. Then I can simply unfollow if I choose. I dont like uneccessary typing is all, I'm lazy and I stuck at typing.