Hi again, and thanks for your additional information about the situation.
I'm surprised the vet didn't do a biopsy to know that it IS cancer, and what type he's dealing with. However, if it has grown and spread on his leg in such a short period of time, the decision to amputate may be the right one. But, I would definitely get a second opinion from a feline oncology specialist. Specialists can be found at ER vet centers and veterinary teaching hospitals/universities, so if you're fortunate enough to have one or both of these facilities in your city, try to get an appt. for evaluation of your cat's paw, there.
If the paw has to be amputated, your cat should still enjoy quality of life and he will learn to walk with only the three remaining paws/legs. You can help him adapt by being encouraging and helping him get around at first, but cats are very adaptable when it comes to these things, and soon he'll think he's walking and playing just like he did before. I'd just like to be sure that the diagnosis you're getting (you really need to ask the vet what type of cancer he thinks this is, and ask more questions about the proposed surgery) is the correct one, and also, more bloodwork should be done, since 6 weeks ago the blood showed nothing unusual. This will give you more information on what to expect:
I hope you're able to get some more information and a second opinion from a specialist, and that your cat will be fine. Would you please keep me posted on how he's doing and what the vet tells you? Thanks.