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I'm sorry for your loss. Unfortunately, this type of scenario is becoming more common. You may sue the VA via the Federal Tort Claims Act. Typically, servicemembers and their next of kin are prevented from suing for injury under a rule known as the Feres Doctrine, but that applies to treatment given while on active duty.
Since your son was being treated by the federal government, and was not on active, you can sue for the malpractice treatment received.
There are some other nuances to it, and it's not something I recommend you do without an attorney.
I hope this helps.
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