Wearing of that cap is covered under the First Amendment as free speech, just like people I have seen wearing t-shirts with the "F" word on it.
I checked with a friend who is a deputy attorney general.
As Hugh smith said, he may support Vietnam Vets.

I noticed he immediately said he was not a Vietnam vet when asked; so I doubt he was trying to pass as one. Many family members of vets where hats, T-shirts, jackets, etc that identify with their kids, siblings or parents.
Did you ask if a family member was a vet?
Gerry Ney