I think Drew has some bad stuff on Shiloh. But now since he doesn’t have his memories Shiloh doesn’t care.
They did know each other. Although very brief, they showed us a picture of the two of them (Drew looked like Jason) in camo with weapons, standing against a wall together. Of course, Shiloh was the one that produced the picture.
Yes, Drew must have known Shiloh, because why else was Shiloh relieved when he realized Drew doesn't remember that time. If Drew did, he would probably know something about Shiloh that is damaging to Shiloh.