I think you are referring to the summer 2007 visa bulletin fiasco when they made it all-current, not 2008.
Sometimes they assign a visa number when the PD is current, and then issue the approval and physical GC 1 or 2 months later after retrogression has kicked in, but that clearly is not the case here. And sometimes they approve the derivative before the primary, but when that happens the primary will get the approval within a few days, maybe a couple of weeks later. That didn't also didn't happen here.
So it definitely looks like they approved his in error, assuming that his GC is not from a family-based petition (parent or sibling) or asylum case that you didn't know about. What is the three-digit category code on his GC? His should be E27 or E39 if it's a derivative EB2 or EB3 green card.
If his GC is an erroneously approved EB derivative GC, you can write to USCIS with as much relevant information as you have (his and your A#, his and your I-485 receipt numbers, your I-140 receipt number, copies of your and his I-485 receipts, etc.) and explain the situation and they probably will revoke his GC, although it may take them months or years to take action. If they don't act on it within 5 years after approval, he'll get to keep the GC because there is a 5-year statute of limitations to initiate the rescission of a GC that was issued in error.
If you are still married when they rescind his GC, they'll put him back into pending AOS status (if they don't automatically do that, his lawyer will surely ask them to do that during the rescission hearing). Then if you get divorced while his AOS is still pending, you'll have to write USCIS again to ask them to cancel his AOS, providing proof of the divorce along with the relevant information to identify the case as I mentioned above. But if you divorce after they reapprove his GC, he'll get to keep it.
