RaZy, if you open up console (by pressing the ` or ~ key) and type "status", the name of every player in the server + their steam ID'd will show up. Copy and paste it.
If, however, he left the game, press 'Shift-Tab' while in game (to open the Steam overlay), then click on your friends list. On the bottom there is a selection to "add a friend". Click that, then click on "view players you've recently played with". Then on the tab up top, click "Recent Games", and his name should be somewhere on the top. Click on "Show Profile" next to his name, copy his steam profile URL, then go here:
http://steamidfinder.com/. Paste the URL, press enter, copy the Steam ID, paste it.