Without knowing the extent of his metastases or mitotic rate, there is absolutely no way you could accurately say that those 6 weeks would not have made any difference to treatment at all. You're right, those 6 weeks might not have been enough time for the cancer to grow or spread; if it is a highly aggressive cancer, however, those 6 weeks could've made the difference between one met and ten. This is why we have forensic analysts whose job it is to study this kind of thing. It is often far more complicated than 'nah, those 6 weeks meant nothing'. The fact that he ended up in the emergency room in enough pain to warrant an emergency CT is more than enough to prove actionable harm and warrant further investigation by authorities. If his mitotic rate is high enough, or if a highly-transmitable system was involved, yes, a doctor could absolutely be sued for missing a distantly metastasized cancer for 6 whole weeks.
In medical malpractice suits based on negligence, the act of negligence itself is given more weight than actionable harm. This wasn't a case of a doctor misdiagnosing someone; the doctors didn't diagnose him AT ALL. They just shipped him off, believing him to be a drug seeker. No diagnostic tests were performed (a test ordered but not performed is as much use as a chocolate teapot) not even a blood panel or an ultrasound. That is what I mean when I say 'basic diagnostic tests', not a CT scan. That is exactly how my aunt's advanced colorectal cancer was diagnosed - she went to the emergency room with severe abdominal pain, the doctor said 'ooh, that doesn't sound good. Let's give you an ultrasound to take a look to see what's going on in there' and boom, masses, masses everywhere. She was 42. A CT scan is not the only means of identifying unusual masses in the abdomen.
I am friends with many doctors. I know that you guys are prone to 'I went to medical school therefore I know way more about everything and you are a simpleton', but please, stop with the patronizing condescension. It is rude and unnecessary. I try to avoid dick-swinging, but your 'let's see if we can actually learn something today, hmmkay?' Shit rubs me right the fuck the wrong way. I have a law degree from a very good school. I would be willing to bet a pretty substantial amount of money that you do not know more about medical malpractice than me. So please, stop talking to me as though I'm a six-year-old who doesn't know how litigation works or when it is warranted.
So to summarize your points: 1. Yes. 2. No. But the doctor should've ordered some sort of diagnostic test in a timely manner, any number of which would've at least given the indication that more testing was required. 3. Yes, absolutely, if the forensic analysts can support such a claim.
Just like you cannot diagnose someone over the internet, telling someone that they do not have a potential medical malpractice suit when they clearly meet enough criteria to warrant further investigation, based on your own bias against such suits, is unconscionable. You are not trained to know when and how Doctors fuck up in a legally-culpable manner. You have a rough understanding based on one or two medical ethics classes back in college, but a little knowledge is a dangerous thing, as they say.