Best of luck on your surgery. I'm sure you'll be fine. I had a lump removed from my left index finger (and I'm a lefty) as a kid and it fully recovered - turned out to be a benign cyst. I'm sure you'll do OK as well.
While there are many deaths and maimings among the military going to Afghanistan, there's far more people who make it home at least physically intact. Factor in that there's no guarantee he'll go there in the first place, and I think the odds will be in your favor. If he does end up there, the biggest thing you need to be concerned with is PTSD rather than physical injury. But I think you can help him in staving that off.
In my non-professional opinion, the best thing you can do for your son is to keep in touch FREQUENTLY. Even if he never writes back. He has to know there is a life waiting for him back home and to keep him connected to that life. This will also help him feel less like an utter stranger when he comes back after however many months and there's been so many changes around him. Those months, if spent in a hostile deployment, will feel to him more like years. Tell him every piddling thing going on at home, from the funny face the cat made to the election for mayor. Scan some articles from the local paper covering local news and send them to him. Shoot little videos of day-to-day life on your phone and email them to him. Do whatever it takes to keep him feeling connected to his civilian life, and I think the readjustment when he returns will be much easier on him. He will return a transformed man - most soldiers return better than they departed in terms of their purpose and discipline - but with help from you, he'll still be in touch with his past.
I have few real fears left in life. I spent too much of my life afraid of all kinds of things, realistic and unrealistic. Through aikido and self-examination with a little Zen on the side, I've become a much calmer person. If death, pain or disaster comes, I may not welcome it, but I am prepared for it mentally and emotionally. It's made me a much happier man, able to appreciate all aspects of my life.
I learned that there's little point in expending energy worrying about the past, because the past can never be altered. Worrying about the future doesn't help things much, either, since no one knows exactly what the future will hold. You are best served by being prepared for all possibilities - with the best way to be prepared being that of remaining mentally still and emotionally calm, ready to respond as needed, when needed, but not a moment before, and without any distractions such as worries in-between this moment and that one. Worry can only serve to cloud your judgment at a time when you need the most clarity. It's not to say I never make plans for contingencies, but if I'm doing everything I can in a given moment to insure a successful outcome, I can let go of worrying about some possible future event before me that may occur. Remaining in the present, connected to THIS moment rather than the one before or the one after, is one of the more difficult tasks in life for many people.