First the GF is the dependent of her parents so she cannot claim the child at all AND you cannot claim her either so she is out of the mix.
Then support is a mute subject since to claim the child all that needs to be true is that the child is not self supporting which I assume he is not at his age.
You will also need to file as single and not Head of Household since you probably didn't pay more than 1/2 of the total household support for more than 1/2 the year.
So we are down to you or the grandparents claiming the child....
You, as the parent, have the superior right to claim the child and if you & the grandparents both claim the child you will win in an IRS audit.
IF you choose to let the grandparents claim the child you can as long as the grandparent's income is more than yours.
Since you all live together you all really need to sit down & talk this over ... see whose return the child does the most on so you get the best return for the entire family not just one person.
