You need to discuss this with him. Let him know it is hurting you.
What do you do? What would you do in any other situation? If something hurts, you stop.
It sounds to me like you've fallen into the trap of thinking that your boyfriend is the one who dictates what happens sexually. Not so! Sexuality is mutual, and you are as much a partner as he. You decide what happens as well.
If it's hurting, tell him. It sounds a though you're very aroused; however, I suspect that he's either bumping against your cervix (at the top of your vagina), which can cause cramps, or he's stretching the outside of your vagina. It's possible that you aren't as wet as you think you are--or maybe he's just a bit too wide for you.
Whatever is the case, you need to tell him that when you're in pain, you will ask him to back off and slow down. Sex is for both of you--not just him. Dr. J