Don't give antibiotics as a preventitve - they may sometimes help, but they always hurt a little. The odds of having complications from preventative antibiotics are high, while the chances of him getting an infection are dependant on you conditions.
If they're pulled out, they will probably not grow back. If they are worn to nubs, they might.
It's usually caused by their nails being ripped out, or worn down. Do you handle it roughly? Pulling them off of quickly wll do it, as will prolonged walking on screen or hard surfaces.