It is hard to answer for two reasons. First, going by standardized breeds, he cannot be Serama x Amer/Araucana cross as either breeds have feather legs.. Feather leg is a dominant trait.
Second is the huge mess of Amer/Arau/EE. It is impossible to know what a person is actually talking about until the birds are either seen in person or with pictures(even with the persons being 'absolutely sure of what they have'...). Could be he had feather leg "arau/amers" which in reality were (feather legged)EE so in that sense it could have been Serama x feather legged EE, which would explain the leg feathers on him.
Seeing how the frizzle hen has leg feathers PLUS pea comb, I could believe a Serama x "his frizzle" being parents much easier.
His weird and floppy pea comb is a normal and very common result of being a cross between pea and single.
It could be possible he actually had araucanas or amers, that frizzle hen could pass for an arau/amer- frizzled cochin hen cross or mix.
there are feather legged seramas, they call them "booted" and there are frizzled ones as well. Maybe the roosters daddy was a frizzled, booted serama, which would most likely tell you he is not a serious breeder, but more of a "gene stacker" which is not neccesarily the same thing as a serious breeder. I am a gene stacker, by the way, aspiring to be a serious breeder.