The thing doctor wanted to say was a central venous line and since it is a procedure (kind of surgery), informed consent is done. A central venous line can also be done at groin region if there is no injury there.
Even as per your description, your uncle's accident is of major kind so the condition is critical and so doctor should get a risk bond consent from the patient's relative.
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You should take your uncle to a center where critical trauma care is available.