In my opinion, your boyfriend doesn't sound like an addict. I see an addiction as something that gets in the way of your ability to function day in and day out, holding a job, maintaining relationships. I also don't think because he felt guilty about looking at pictures makes him an addict either. I'm sure plenty of addicts don't feel any guilt at all.
I think what is far more concerning is the lack of trust in your relationship. You don't trust him, and even if he hasn't said it, he probably doesn't trust you since you've gone through his things. I'm not trying to cast stones; we've all had our weak moments and made bad decisions. But what I am trying to get at is that you guys need to work on rebuilding trust and learning to communicate with one another if you want this relationship to last.
I think you did make a good point, though, in realizing that he's done a lot of maturing in three years. I also agree with those telling you to go with you gut. You have to do what you think is right.
And besides.. The way I think about it? I've probably looked at a lot more girls in bikinis and lingerie than my husband. It's called online shopping

End result is still the same