There are anal sex toys of different girths you can use to practice loosening yourself up. Go to sextoyfancy or any of the good sex shop online and look at the section of anal vibrators.
Start with a small one, say 3/4 inches around. Have him lick and tease you, whatever foreplay that gets you hot, and once you are very excited, slowly slip the vibrating dildos in. Just let it sit in there while he gives you a nice orgasm. This way, you will learn to associate the "full" feeling with the pleasure.
Once you've gotten used to the smaller toy, move up to a lager one, and take the same steps.
This will help loosen up your mind as well as your muscles. It may take a bit of time and coaxing.You can use toys for a few weeks before any attempt for him to fully enter you. Even then, when you work your way up to him penetrating you, take the same steps- get excited, let him go inside, but he shouldn't move- he should let your body get used to the feeling of his penis before he attempts to move at all. The first few times he should pull out after your orgasm, and then you can have PIV sex for his orgasm. After you start feeling confident that you can handle his penis, he can slowly move. And remember- take a DEEP breath, and RELAX.