We all hide certain aspects of ourselves when we start dating someone new, because opening up requires trust -- and trust requires time. While open communication is important to a long-term relationship, some things are better left in the dark for awhile. So until it gets more serious, your dude is probably going to not be super upfront about...
His real number of past sexual partners
Although he may brag (or lie) to his friends about it, he'll likely avoid the subject with you entirely. You're together now and that's what's important, right? As long as he's 100% upfront about STDs and testing, anyway.
His internet history
Giving you access to his unlocked phone or laptop would be like letting you give him a shot of truth serum; it'd tell you pretty much everything about him -- and not just his porn-watching habits. Maybe he's self-conscious about his shopping ideas or questions he asked Google at 3 a.m., like "Can you break a boner?"
Even if he does turn his password over, a new boyfriend is usually diligent to delete his browsing history, for fear that you'll delete the relationship. And don't be upset if he doesn't let you see what he's texting -- he's probably not cheating on you; he just wants to feel like he still has a little privacy.
How disgusting his bathroom is 99% of the time
If he knows you're coming over, the bathroom is the absolute first thing he'll clean, lest it haunt you forever. Also, believe it or not, your new guy is holding in the vast majority of his farts around you -- the pain and discomfort he goes through on an average date is immense; he likes you that much.
Guys spend money on dumb crap all the time -- even if he doesn't consider himself a shopaholic, the fact is that he can't pay rent because he bought a $500 video game system and too many Wolverine statuettes, and at some level he's aware that the fruit of his labors shouldn't just be "Transformers" t-shirts.
His parents support him
A little help from your parents now and then is nothing to be ashamed of, but every guy wants to seem independent. That's why he's not going to tell you that his mom still does his laundry. Don't worry, as soon as you meet them, they'll reveal all of his secrets and the jig will be up.
His ex-girlfriend memorabilia
Guys deep down are sentimental creatures too. That's why sometimes we keep stuff from our past relationships. Finding out about these things can potentially make a new girlfriend wonder, "Why do you still have this? Are you not over her?"
For the record, we can be over someone and still have a fondness for our time with them. If the day comes when you two move in together, then yes, maybe it's time for him to throw out that movie stub, but for now, allow a guy his memories. (Unless, of course, he has a collection of ex-girlfriend souvenirs taped to his wall. Shudder.)
His idiot friends
He thinks a person named Toenails Teddy is the most hilarious dude ever ... but he also knows meeting this person would make you question why he keeps such weird company. He'll introduce his buddies slowly, but until then, date night and Toenail Tuesday will be two different evenings.