Sleepover Phobic

What do I do with a 9-year old who won't go on a sleepover? Last weekend he was all excited to sleepover with his friend, but when we got there, he threw up. We thought it was a one shot deal, but it wasn't and he wound up being driven home with the stomach flu. My plans to go to a friend's birthday celebration were thwarted and I spent 24 hours with a bowl catching barf. The same family has invited him over this weekend, and yeah, I have to admit I was excited because O is away for the weekend too. I have never once had a night when both kids were at a sleepover. But now he doesn't want to go. He can't tell me why and insists that it is not because he is embarrassed about last weekend. Just doesn't feel like it. I know I can't force it, but this was supposed to be practice for a week-long camp he is going to in July. Getting used to being away from me, in a safe, easy, fun way. Do I force him to do this, knowing it will help him in the long run? Or do I let it slide, thus enabling the phobia? I suspect its a no-win no matter what. And that night off? It was wishful thinking.