The fact that other partners are invited makes it rude; I think the first thing you need to do is ask if there's any particular reason he isn't. Perhaps there's something you're not aware of that needs sorting out? If they say it's a financial decision then I would decline my invite.
If they can't give you a decent explanation for his lack of invitation then I think you're under no obligation to give a decent one for your not coming.
Don't feel you have to lie, just say sorry, I won't be there.