Boyfriend started crying during sex...

We are both 18 and have been together for 4 months. He had been really wanting to have sex and we ended up doing it in the back of his brothers pick up truck. Ik... Weird.... I was a virgin, but in the middle of sex I noticed a tear. I asked was he okay and he said yes.... Then full on water works and he got off me and started crying. I sat there and gave him a hug. He said "I'm sorry. I'm pathetic". I just shushed him and kind of held him and told him it's ok. He hasn't spoken about it and I just don't know what I could have done to make him cry.