There are a lot of reasons kids will say things to upset others. He's probably having as hard of a time as you are with the whole situation. Try calmly correcting him, without escalating. If he continues, you can say "we're finished talking about that, let's play xyz...". If the statement doesn't get attention (positive or negative), he'll probably move on. Good luck!

Yes, thank you! Sure, it'd be great if people remembered arithmetic rules, but just write it better and it won't matter.