If It was me I would lie yeah my memories are back

I dont think he should lie since that can be proven pretty fast that its a lie with a few questions what he wont remember

What i think is he should have told him he is starting to remember bits of them and to give him some time to remember everything or something xd like that would make sense

I doubt that he's realizing that these are his memories

I think he does it just doesnt feel real to him, like imagine not remembering anything and then seeing a few memories randomly surge up, since you dont remember it doesnt feel real to you since you dont know how you got to be like that you dont remember what you went through to be in a relationship like that with someone else just see that you guys are supposed to be together apparently because thats what everyone tells you and you see too in a few memory flashbacks
I can see why he wouldnt be able to explain these things to himself and still be confused especially with his trash personality