Is This the Militia?

Joe Blogs, a dual US-UK citizen is a member of the Rifles, which is a direct descendant of the Royal American Regiment, which was created in 1756 as the 62nd (Royal American) Regiment to defend the thirteen colonies against attack by the French and their native American allies.
This predates the constitution. It also fought on the Loyalist side during the War for Independence.
Does he count as being a member of the Militia for Second Amendment purposes? Seriously, wouldn't this regiment count as being the militia?
Likewise, does a member of the crips or other street gang count as the militia for Second Amendment purposes?
What makes one a militia under the Constitution? Wouldn't it require being formed under Article I, Section 8, Clause 16:

To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;