I'm sorry you are in this spot.
I do investigations here where i work and if we have a staff member who is under investigation and does not want to cooperate, they are fired and guilt is assumed.
I dont know how this works in the military, but i would assume not cooperating is not an option.
I dont mean to scare you, but i think he does know and is not telling you.
If he doesnt cooperate, what will the militart do to him?
They arent going to just not let him cooperate and drop it. you know?
I guess a polygraph is a good option.
Take care bw, hugs to you