Why his parents do not satisfy those 5 tests, right?
You may want to say his parents do not satisfy the 3rd test, citizenship test, right? Since his parents stayed in US for the whole year, for tax purpose, they are residents of the US. Now you should know why usually visitors only got less than 6 months' visa. I remember I read somewhere, as long as you have a valid visa and stayed more than 6 months, for tax purpose, you are a resident.
Why don't you give irs a call? They will give you the official answer.