You look at men and notice if they are attractive or not - yet you don't think about them while you're with him.

He looks at women and notices if they are attractive or not - yet he doesn't think about them while he's with you.

He's been telling you that he does the same thing that you do. You either trust him, or you don't. He can't do anything else to convince you otherwise, and no one else can convince you otherwise.

You're absolutely correct that it's your own insecurities. Which means that this is something which really, only you can solve. Remind yourself that he is with you, and has chosen to be with you for two years.

You're mind-reading, and you are believing your own insecurities over him. You cannot read his mind, so stop validating your fears. Each time you think he's lying, you need to remind yourself that you have NO reason to believe that he's lying and that it's your own insecurities playing tricks on you. Seriously, just tell your brain to eff off every time those thoughts pop up, and don't act on them.

What's going to eventually happen if you continue asking him and not trusting him, is that you are going to push him away. He hasn't done anything to deserve your distrust, and eventually he's going to want better than that, because he deserves to be trusted.