Has the doctor discussed with you that a child's heart will be smaller than an adult's heart metrics? It seems if the heart grows as well as the circulatory system, the valve orifice may not provide a sufficient opening (stenosis) later to provide enough blood into the system with each heart beat.
A bio valve would need to be replaced in about 10+ years and at that time your son may have reached full growth of the heart. Won't need medication as with a mechanical, and there may significant changes that favor a better decision at that time.
First of all, I am so sorry for your family's struggles and your son's illness. Nothing is more difficult than worrying about the health of a child. If your son does require valve replacement, his surgeon will most likely recommend a bio-prosthetic valve (pig or bovine) as opposed to a mechanic valve. Mechanic valves require a daily dose of blood-thinner, which means the person taking it has to be extremely careful to avoid internal bleeding (no accidents, falling down stairs, contact sports, etc), which might be a difficult lifestyle for such a young person to adjust to. The problem with bio-prosthetic valves, however, is that they need to be replaced every 10-15 years, sometimes sooner (my mother, for example, had a porcine valve put in two years ago, and is having it replaced again next Friday. Needless to say, we're not happy about it). But, i understand this is a very rare scenario). Mechanic valves can last up to 30 years, but, because your son is so young, he will need to have repeat surgery, regardless. The best thing you can do for your son is ask the tough questions, and find a surgeon who is going to take every single one of your concerns seriously. It isn't an easy choice, but whatever you decide, I'm sure your son is in good hands, and will be fine.