where is the mistake???

Hi! What is wrong with these sentences: "Is John ill? He has lost lots of weight.
Yes, he is rather slender these days, isn't he?

I know there must be something wrong with second one but I have no idea what. Can't we use "rather" with adjectives of second degree like slender? Other than that I have no idea, really. Please, help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[river]

Slender implies a generally attractive and pleasing thinness. (dictionary.com)

[magdalena]

Thank you!!!!!