Hello, This condition is called hemihypertrophy where all or part of one side of the body is larger than the other. This is likely something with which your son was born. The telangectasias (spider veins) suggest that he has associated vascular (blood vessel) malformations. He needs an abdominal ultrasound first of all to make sure that none of his internal organs is also affected, as well as Doppler flow ultrasound of the blood vessels in the affected leg.
Concern later in life depends on what is found on further testing, eg is his liver normal in size and shape and blood flow. Would not worry about loss of limbs. There is nothing to reverse this. He can play sports.
Please ACCEPT if I've answered your questions, and post back as much as needed. Best of luck,