I don't know how he got the rose comb. His mother is a good quality sizzle with a walnut comb and his dad has a straight comb.
For the comb, you have two basic genes, Pea and Rose. There are some modifiers so you can get the buttercup and the other weird stuff, but concentrate on the two basic ones, Pea and Rose. If a chicken has both the Pea and Rose, you get Walnut. A single comb means that chicken does not have either the Pea or the Rose genes.
So his father did not contribute either a Pea or a Rose because as a single comb bird, he did not have any to give. The mother had both a Pea and a Rose since she is walnut. But she obviously is split for the Pea gene because she did not give one to her son. She may be pure for the Rose or she may be split for the Rose. You don't know. But the mother gave him a Rose.
I don't know what the comb strandard is for a sizzle or a silkie for that matter. But that cockerel has pp,Rr for comb genetics. If you are breeding for show, you might want to consider that.