All testified about him, and wondered at the gracious words which proceeded out of his mouth, and they said, "Isn't this Joseph's son?"
And in the Greek, the if is in the subjunctive, rather than in the indicative case. And the if in the subjunctive is not a question, but is a declaration, "Since thou art the Son of God". He isn't really questioning the fact if Jesus is the Son of God or not, but he is in reality saying, "Since thou art the Son of God,"