My new kitten looks alot younger than 6 weeks + wont eat/drink much?

Got a kitten today from a friend of a friend..He's very small compared to my 8 week old kitten, i'd say about half the size, maybe a bit bigger. But I was told he was 6 weeks old, and ready to leave his mother. I'm now having doubts about this kitten though..Because he's only drank a tiny bit of kitten milk, once, and the lady said she was feeding him on adult cat food, an when I tried him on kitten food, he hardly touched it, just sniffed a bit and maybe took one or two licks of it and walked off..He's very timid and is currently hiding behind the hoover..he also keeps miaowing and looking around and im really worried that he's actually trying to look for his mother? I don't think he was ready to leave his mother. Trouble is, the lady has already sold the mother cat..