Hmm, in my opinion, that's what there is doesn't sound very natural, but it doesn't sound flat out wrong either (I'd understand what you meant). I'm guessing most people would phrase this as that's all there is and this might imply something different than what you're looking for.
That's all there is implies that there's nothing else to eat and that's why they're eating spaghetti and meatballs. My guess is, however, that Mom has already started cooking or planned dinner, and she does not feel like rearranging her schedule or planning something else just because her son isn't in the mood for it.
In this case, she'd most likely say something like "Sorry, that's what it's going to be" or "Sorry, that's what we're having." I'm sure there are numerous ways to rephrase/express this though.