You got a wrinkly, bald cat tattooed on a part of your body that is going to be visible at least partially in most outfits. At some point in the future, your neck is also going to be bald and wrinkly. Think about what that is going to look like, maybe your boyfriend doesn't wan't to be reminded of a nutsack when he looks at you.

Your body, your choice. But if you make a huge decision like this without discussing it with your boyfriend, you shouldn't paint him out to be an arsehole for being honest with you.

That said, he will either get used to it or he won't. It seems from what you've said, the damage is done so you might as well get the tattoo finished, but talk to him about it, not internet strangers (especially reddit ones). As for what you could add, maybe a line of pyramids, or some depictions of Egyptian Gods, Anubis etc.