I googled him, and it seems his veteran status is not in question.
http://militarytimes.com/blogs/battle-rattle/?s=shamar+thomas
Then Shamar Thomas has joined the Senator Kerry school of United States veterans.
Kudos for Shamar Thomas standing up for the rights of others, but he needs to do it out of uniform. All conjecture aside, these are the rules:
DoD Instruction 1334.01- Para 3.1.3 The wearing of the uniform by members of the Armed Forces (including retired members and members of Reserve components)…is prohibited when participating in activities such as unofficial public speeches, interviews, picket lines, marches, rallies or any public demonstration…
Para 3.2 Former members of the Armed Forces…may wear the uniform in the highest grade held during such war service only on the following occasions and in the course of travel incident thereto: 3.2.1 Military funerals, memorial services, weddings, and inaugurals. 3.2.3 Wearing of the uniform or any part thereof at any other time or for any other purposes is prohibited.
He (and any other service members who see this post) should know the rules and should have been briefed on these military laws. His opinion is his own and by wearing that uniform he is, by the letter of the law, implying Service sanction of the cause for which the demonstration or activity is conducted. He is thus subject to the articles of the UCMJ.
( Comment from your site reference )