[DarkSwan]

Just be open. Honesty is always the key to any relationship. Tell him you are nervous about taking that next step. He already knows its not "normal", but I wouldn't make comments about it of it embarrasses him. I'd ask him how he'd like to discuss it, and find a time to have an open discussion with no worries about what is said. But always be kind. There are ways to phrase things, and ways to not. So think before just blurting a question or answer out.

My 1st boyfriend was large. Over 9 inches erect
It wasn't particularly comfortable at times, but for reasons beyond his penis size. He was very abusive. Providing your boyfriend is not, and listens to you, then when you do take that next step, simply tell him if it's hurting. A good partner will change what they are doing.