If you don't know why you are jealous of him, other than you know you don't have anything to be jealous about, then the problem is with your perception of him not being trustworthy, isn't his problem, it's yours.
Any relationship will burn out quickly when one party is subject to unfounded accusations.
You need to change your thinking.
When you have a thought about him being dishonest, or untrustworthy, instead of finding reasons to justify it, and going down that slippery slope of ruminating about it until it explodes, deal with the thought.
Stop yourself, and ask yourself, what is it I am afraid of. What has caused me to think this way. Accept the thought (he spent time with so and so, or what ever it may be), and find another answer, instead of building a case of condemnation against him. Find the thought that triggers the subsequent action, before it gets out of hand.
If he says he's going out with his friends, and you get that familiar feeling of 'something more is going on here', before you respond to him, ask yourself some simple questions. Does him going out with his friends mean anything other than he's going out with his friends? Do you have reason to believe that his actions are going to affect your relationship with him? Is his going out going reason for you to presume, and/or assume he's up to no good?
If the answers clearly add up to 'no's' and no 'yes's', stop yourself right there from making a mountain out of a molehill. Direct your insecurities instead to allowing what you have reasoned to be non threatening or harmful in any way, instead to doing something else while he is gone. When he comes home accept that what you have reasoned yourself, will happen, and that nothing is wrong, and congratulate yourself for having control over your thoughts.
Each time you stop a negative, unfounded thought, you will free yourself from the grief of not allowing yourself to trust him, and he will realize that even the simplest actions on his own, aren't subject to interrogation.
It takes work, but just remember, if you have the ability to put a whole story behind anything he does that is always negative, you also have the ability to change your focus, deal with the thought, and direct it in a different, more realistic way.