This is without a doubt a war crime
There are many doubts. He could be faking injury, drunk, still armed. The observer drone who suspected incapacitation was not the one attacking so you'd have to prove the actual attacking drone operator believed the target was incapacitated. Injured =/ incapacitated and his presence alone at the front occupies enemy territory which means they are a participant.
The bounds on what are 'reasonable' are very different in war than civilian life. This tendency to jump to 'ope warcrime' is as irrational as saying someone with an automatic weapon firing a burst commits a warcrime when the second round in a row strikes the victim. 'They were injured with the first bullet so it is a warcrime and automatic weapons must be prohibited by law'!
Had this soldier been on a stretcher, and/or being attended by an unarmed someone with a red cross on their arm, or had waved a white anything in the air, I'd agree with calling it a warcrime. A still conscious soldier being tossed out on their ass by 2 armed soldiers and 'appearing injured' in this era where the enemy feigns injury/death to avoid drone strikes is not hors de combat.
In a combat zone you are considered combat effective until proven otherwise. This is not a warcrime. Faking an injured leg or being dead are commonly done to avoid getting droned. Not seeing a weapon isn't an excuse either. Grenade on the belt, or sidearm in the coat are both likely possibilities. You don't know and until you do it is safer to take them out then apply warm and fuzzy civilian sensibilities to the problem.