Yes it is correct but not the right translation. If you would ask "Is that his dog?" it means "Ist das sein Hund?" in German not "Ist es sein Hund?".
The full speed sound of this is impossible to make out as "ist es sein Hund" Sound like ist das ein hund
It's not English. The sentence makes no sense. There are a few things wrong with that sentence. But the main problem is there needs to ve an apostrophe. it's dog.
Could I say "Ist er sein Hund?", using 'er' instead of 'es'? Considering 'Hund' is a masculine noun, would that be right?
I think it's not, because the verb "sein" doesn't work with the accusative form. I'm not sure if this is the right explanation, but when you are using the verb "sein", the "Hund" (dog) in this case is the subject, and not the object.