"It is the boy's dog" should be an acceptable answer

It's the boy's dog would probably be translated like this: é o cão do menino

what is the difference between cachorro and cão?

No mistake here. "It's the boy's dog" and "the boy's dog" are not interchangeable with "The dog is the boy's," regardless of the language.

"It's the boy's dog" = é o cão do menino "the boy's dog" = o cão do menino

I put the "the dog is for the boy", why isn't that acceptable?

I had put down that the dog belongs to the boy....what do you guys think?

Poor decision not to allow perfectly acceptable alternative!