Hi, Sam, welcome to BYC. I believe that he is a dark Brahma cockerel. They tend to mature slowly - that would explain the fact that he has not started crowing.
I agree it's a dark Brahma cockerel. He's only 4 months, he's still just a baby. Brahmas, being a giant-type breed, often don't mature until a year or so. He's going to be beautiful!
He looks like a darker version of our light Brahma cockerel. Ours is four months old and huge... larger than any chicken we've ever had. He hasn't started crowing yet, but is flighty and stand-offish. Good luck with yours! He's beautiful