One thing jumped out at me while reading your post, that I want to point out in case others over looked it. It's also possible I am not interpreting it correctly.
He would be a qualifying child apart from the condition of not staying with me for more than 6 months
It's my understanding that if you do not report the student lived with you "the whole year" then you the parent don't get any education credits for that student. Be aware that time spent away for the primary purpose of attending school is considered as time the student lived with you. So you probably should be indicating that the student lived with you "the whole year".
As stated before, once your tax liability gets to zero, that's it. You can claim all the additional dependent credits you want. But you don't get refunded back to you, that which you never paid in. (don't confuse the dependent credit with the refundable portion of the AOTC either please.)
