1. - you have evidence of him being dishonest in the past and are finding it hard to trust him now
2. - you DO NOT have evidence of him being dishonest in the past but instead have your own insecurities.
With the first one, if he has been dishonest with you in the past and you are finding it hard to trust him again then once again, I think there are 2 things you can do. The first requires him to basically disclose everything that you ask of him. As you have been hurt before by his dishonesty, you may feel the need to know about even little things, so you can start to gain proof that he is being honest and you can trust him again. If he is not happy with this and does not want do that for you, you need to ask yourself if he really cares about the relationship as much as you do and what future it holds. The second, is that you don't ask for details about everything, but instead forgive him for what happened and trust him again. Obviously with this option, if you do that and once again find out he hasn't been honest with you, then it could be the end of the road (which isn't necessarily a bad thing).
If he hasn't been dishonest before (that you know of) and you are just having trouble trusting him, then you need to take a look at yourself and think about why you don't trust him. Is it something from your past? Someone else you know of that you don't trust? If it is to do with something else and not him directly, you need to realise it's unfair to put that mistrust for someone else on him. I'm guessing if you talked to him and asked about things in a non-obtrusive way, you'll find that the communication lines will open up and you'll feel a lot more trust in the relationship.
I hope thats of some help and you are able to get past this. Remember, relationships are meant to be fulfilling and enjoyable, so start enjoying!