His name is Čeda.
When you talk about him with someone else you say Čeda. "Čeda is smart, Čeda sings good, Čeda this Čeda that".
When you are takling to Čeda you say Čedo. "Ćedo come here, Čedo be good, Čedo do this, Čedo do that".
If (male) name ends with
a, in second case
a goes to
o (Female names does not change).
If name ends with
o, in second case you do not change anything.
If name ends with any other letter in second case you add letter
e to name.
So you have Čed
o, Igor
e, Željko.
The exception is a male name Nikola, in that case you do not change anything (I'm not sure, maybe there is more exceptions).