He must be sf grey.
If he would be df grey then all his chicks would be sf grey and all his offspring would be grey (blue grey, green grey)
but because he have blue offspring then he is sf grey.
here are the calculations from a sf grey normal male (with no other color adding factor)
and sky blue cinnamon normal female
% from all 1.0 (boys)
50.0% 1.0 grey(sf) blue /cinnamon (grey males split cinnamon)
50.0% 1.0 blue /cinnamon (blue males split cinnamon)
% from all 0.1 (girls)
50.0% 0.1 grey(sf) blue (grey females)
50.0% 0.1 blue (blue females)
I love my budgies
Thank you FaeryBee for the signature