I'm 29 years old with a VM throughout my left elbow/forearm, when I was about 10 or 11 doctors told my parents that my arm should be amputated (I wasn't diagnosed with a VM at that point) my parents REFUSED, and I couldn't thank them enough for it!
I know it's a very frustrating thing to deal with and at times a very painful thing to have to deal with. My advice is get another opinon! I would recommend Dr.Wayne Yakes (my doctor) he is in Colorado (I'm not!) but he is fantastic. I believe his contact information is on this site. You can send him a CD of any MRI's or other tests that your son has had and he will let you know if he can treat it.
I beg you that you don't let them amputate your son's leg until you have checked every resource possible.
If you have any questions don't hesitate to ask me.