Why does my boyfriend have this anxious/sick feeling?

My boyfriend and I have been together for 8 and a half months. We love each other, but every so often he (out of the blue) gets this sick/anxious feeling in his stomach about or relationship and future. He can't really explain to anybody what brings it on, or why it is caused. He says it is because he is so scared to lose me (he has a tendency to over analyze things and worry about what could happen in the future, whether it be good or bad). We've promised each other that well always be friends, no matter what (yes, I know, that doesn't always work out) I don't get this sick/anxious feeling at all, that's what makes it so hard. We've decided to take a break so he can figure stuff out, but I still wish I could understand why this is happening. He's told me that the thought of dating other girls besides me makes him sick, and he's not ready to give up on our relationship.

Its just hard because I constantly tell him how much I love him and how happy he makes me, but it never seems to help the feeling he has. Neither of us have been in a relationship that's lasted this long, so could it possibly be that he is just scared to commit long term (maybe he thinks he's missing out on experiences and other girls?) but at the same time loves me to much to let me go?