A different kind of first time.

So my partner and I have been talking for about a year. Sleeping together about 7 months. He isn’t always vocal about wanting to try new things. I tell him I’m up for just about anything but if I’m not then I will let him know that I’m not that comfortable yet. Anyways he’s been wanting back door and I’ve always been against it. Well now I want to try it more for him than anything. But I’m a little nervous, he’s tried once but I tensed up so he stopped. Any advice on trying that for the first time???