It's serious, but on the other hand this has become a routine operation. You mum or dad should be able to tell you what the doc said the risks were and of what. The mortality (people dying) during or just after this surgery are VERY low.
You can be worried, but don't get all nutty because of it. This is one of the few times when your dad really needs you, so just be supportive and try to stay calm. Visit him as much as you can and ask what you can do to help around the house when he does return home.
It's always a shock
when you realize your parents are mortal and may not be around forever.
It sounds already like you're close to your dad, but this may just be an opportunity to get to know him better.