While these boys do not look pure Silkie, they could be. I would say they are at least 87.5% Silkie (based on mother being 100% Silkie and father bring 75% Silkie, both assumptions on my part). No way of knowing what, if any breed the rest of your rooster may be.😊
He has Silkie feathering. He may be pure though just poor quality with a single comb which is recessive in Silkies. Does he have five toes and black skin?😊
If he is a mix he is at least 75% Silkie in order have silkied feathers.
Do you have photos of his sons? Do you know the mothers of his sons?