Rugbypup, don't give up! And when your guy starts to lose interest, give him some amazing head, or rim and finger him. There are tons of great sex options beyond anal. I find mixing massage with oral and masturbation can be very satisfying for both of you. With all the great tips above, I think Goose Eggs was very on point in saying that everyone's ass (and dick) is different. Maybe at this point you can only try having him penetrate you once a week, but why not use dildos on other days? They will help you gradually be more comfortable accommodating larger and larger things in your arse, until he can get in there comfortably for both of you.
I imagine it is super awkward for him because no one wants to hurt the person they are trying to please. You should make sure that you are talking with him about it, and stroke his ego reassuringly. "I'm so turned on by your amazing, huge dick. I am working hard to be able to take it. It might help me get there faster if you could try this on me" as you hand him a dildo, and let him go to town with it.
If you like this guy, don't give up. And make a pledge (as he should, too) that you are both responsible for the other's pleasure. Do what you must to put a satisfied smile on the other person's face.