It's easy to tense up and clamp vaginal muscles so tight penetration is next to impossible. The fact that he can insert fingers is a good sign and what you need is to go very slowly, with lots of lubrication and take your time. I'm not suggesting you get legless, but a couple of glasses of wine would help you relax a little and when he tries to penetrate you take slow deep breaths and try pushing back at his penis with your vaginal muscles gently. It sounds stupid, but it's the oposite of what's probably hampering penetration and may well help.
I suggest you spend time with him gently inserting one or two fingers and relaxing before you try intercourse. No need to hurry.
Are you worried it will hurt? At the worst mild discomfort if you haven't used tampons and have a hymen to break....penises hurt less than fingers to be honest because they're blunt and smooth and nail free LOL
Remember vaginas are designed to allow a baby through...and even the biggest will can't match up to that!
Good luck
