Jilly, you CAN NOT prevent every owie, scrape, gouge, bruise and cut your little guy will encounter. It is not physically possible, even if you hover over him every moment. Bodies are designed to heal, because they will need to.
If you protect your son too much, you will cripple his life. He will become unable to think or evaluate situations for himself, to understand the limits of his own physical surroundings and ability, or to deal with pain. His immune system actually needs to be challenged from time to time by a variety of germs so it can identify them, create anitbodies, and protect him in the future. In fact, kids who are raised in surroundings that are too clean may never develop a proper immune response, and the idle immune system can't learn friend from foe. Those children have a much higher likelihood of developing asthma and other auto-immune diseases later on, and those illnesses can be disabling in the worst of circumstances.
It's really hard to watch our tender and perfect babies get hurt. But it happens. I'd almost be willing to say, it MUST happen. Take reasonable precautions, and know you're doing the best you can.