So far I have this, but I am not sure if you are allowed to do this or if it is correct:
I said since $X$ and $Y$ are independent and follow the same distribution, they must have the same expectation so:
$E(X|S=s)=E(X|X+Y=x+y)=E(Y|X+Y=x+y)=E(Y|S=s)$ since $X$ and $Y$ are i.i.d. Then, can I say?
$E(X|S=s)+E(Y|S=s)=E(X+Y|X+Y=x+y)$
and since $E(X|S=s)=E(Y|S=s)$; $E(X|S=s)+E(Y|S=s)=2E(X|S=s)=x+y,$
then $E(X|S)=\frac{x+y}{2}$.
This is my steps and logic, but I am not sure if this is correct, so any feedback on my work would be greatly appreciated. Thanks!