You can consider him a position 3 imo, though he can be 2-4. Your damage and worth isn't from items (Unless you go SB/Veil/Dagon) it's from completely fucking over anything the enemy tries to do and never letting them kill you or your teammates or get the jump on you since you have a 2000 range aoe 5s silence with 50% uptime. Your goal is to not die in fights and disrupt the enemy when you aren't killing everything on the map and having complete rune control.
So build whatever items dick over the enemy team the most.
Kill as soon as you hit level 3 or 4 and keep killing until you have your Ult. Then go kill someone in whatever lanes needs to die. Repeat every 90s.
You should be the most annoying thing in the entire world and make them not want to come within screen distance.
I've played him mid, support in lane, primary farmer in lane, and position 1 because of farm. Learn to do the combo quickly, how to kick rocks on reaction, and to always silence.
Also I'm just going to keep posting this: (I think my Solo MMR will be ~3500 solo FWIW I started 5 months ago)