Well, I'd talk to a good lawyer, but even still I don't believe there's any ironclad way to shield your assets, even with a pre-nup. Precedent is there to shred it in divorce court.
Ultimately, you'll have to decide if marriage is worth that risk.
For those saying "if you're having doubts about her now, the relationship is doomed", what are you talking about? The guy is just being pragmatic, not questioning his relationship. He obviously loves her. But even the best of relationships can go sour. People change over time. What they want out of life changes. And otherwise good people can get pretty nasty when they feel like they've been wronged and hurt. So he's smart to try to shield himself from that potential eventuality.