One guideline that I once read was "if you can feel the ribs but not see the ribs, then the weight is ok".

Cat weight can vary so much by breed. If Elliot has some siamese/oriental shorthair in him, then he will always be a naturally thin cat.

I know that neurotic feeling sometimes regarding vet visits. But I usually find that my peace of mind is worth the cost.

Good luck to you and Elliot.