Yep, you can only have O and Neg genes - if you had an A and a Pos gene, you'd be A and Pos, since those genes are dominant.
You still can't assume that your DH is AA, unfortunately. One of his parents could still have been carrying the O gene, which wouldn't show up, since it's recessive, and he could still have an AO blood type (effectively A, but it means that your children could still possibly end up getting the O gene from him, and then they would be O). Since neither of his parents was O, the chances are better that he's AA, but you can't tell for absolutely certain.
You can assume that he's +-, yes.
And your figuring did still come out right, so none of the rest of it really matters except for interest's sake.
Unfortunately all I know about the ABO incompatibility stuff is what I've read in this thread.
I'm A, so it doesn't help me - but it's certainly interesting.