He says he doesn't trust me completely

[Cheeseburgler]

Basically he says I don't always tell him things on purpose.

Last thing was a sexual dream I had (me masturbating in the dream) which I only told him today about. The dream had occurred before last weekend.
These things he talks about that I didn't tell him are all things that I ended up telling him anyway. They weren't things he found out on his own. He got to know because I told him in the end. If I had meant for him to never know, he wouldn't know.

Anyway, he says he can't trust me completely because he thinks I might hide stuff so as not to upset him.

How do I tackle this?

That sounds really very worryingly possessive. I would be extremely careful. And single.

This started badly, he's never going to trust you completely because of it.

I would give up now tbh.

Bin.

You don't sound well suited. He wants to control your thoughts and you want privacy. It will never get more relaxed. Cut your losses and find someone else.