If you aren't 100% positive that he's clean, you shouldn't be doing anything with him. This is your life we're talking about, him thinking you don't trust him is a tiny concern when compared to your life or quality of life. This is something you cannot be too safe on!
Think about this. Say he does have something, he's been lying to you about the other girl, and you don't press to find out the truth. If you end up with some kind of nasty std, is it going to be enough to say "i didn't want him to think I didn't trust him."
the point is, obviously you don't trust him entirely, or else you wouldn't be wondering. How long have you been together? Has he lied to you about other things? Do you know him well enough to tell if he's lying?
My suggestion would be that you sit down with him and tell him that if you're going to be sexually active in this relationship, you're going to be responsible and mature about it. Tell him it's not personal, but you would like him to be tested, because regardless of what he says, you need to be sure. If he gets upset and brings it up as a trust issue, tell him it's not about him, it's about you; and that you are only taking responsibility for your own health.
It'd be better to find out he's got nothing and was telling the truth, than to blindly trust him and find out he's a liar.

Yes, you can get stds from oral sex.

Yeah, you can get std's from oral, but if you haven't had anything "show up" that doesn't mean he is clean! For goodness sakes, there are std's that you can get from oral that may not show up for years, and that may not have symptoms, when you would have symptoms from vaginal sex.
Just because he says he wasn't with this girl doesn't mean he wasn't and you don't know that he wouldn't lie to you. You may trust that he would't lie to you, but you don't know it. I agree with the first post, you really need to be careful and sort your priorities. Don't worry about him getting angry or thinking you don't trust him, all of that is very petty in the big picture.