Zephaniah Adriyen, I don't think this is the opponent trying to cheat unless he specifically tries to impede you doing the same thing, i.e. he gets to shoot your units by sword or muzzle tip, back banner, or other things, like aerials or data shunts standing/hanging off models, etc., or if he claims that just because you were able to be shot by seeing that, you can't use the same LOS
to shoot back.
He does sound like maybe he wants something different out of the game than you/just enjoys a different way of playing, and that requires some discussion in person.
Personally, I can't see playing a game as abstracted as 8th with the simulationist rule of True Line of Sight, that "breaks my immersion" of the game and makes me think that actions such as shooting are only taking place at discrete moments of time after "all your models have stopped moving for a moment" rather than constantly being in motion, firing when opportunities present themselves, etc. Heck, "I go, You go" breaks the immersion of combat by itself and to me, any unit that can FLY (as in the Keyword and it's movement capability) shouldn't get the benefit of cover unless they specifically didn't fly in the previous movement phase and gets to fire on you at the halfway point of your movement even without LOS
(because damn near anyone should be able to shoot at something jumping up over ruins, buildings, etc.).
That's just me though, and I understand that others feel differently, so I have to talk to them about how they want to play the game if I want to satisfy my personal "immersion" concept. Most times I will have to provide my own immersion though.