Just a FYI for parents:
http://www.stoptheclot.org/news/article126.htm
http://www.everydayhealth.com/news/downside-of-low-heartrate-athletes-who-get-blood-clots/
http://patientblog.clotconnect.org/2011/03/02/athletes-and-blood-clots/
Turns out that nobody is immune to blood clots.
It's true. In April I thought I had a pulled muscle in my calf; it turned out to be a clot, diagnosed through ultrasound. I had no risk factors, nothing. Seems that a folic acid deficiency might have been the reason. But the doctor said anyone can get them at any time.
Marie, make sure the doctors schedule all the necessary blood tests to check for missing proteins, mutations, and genetic factors which might make his blood more likely to clot.
Going to keep you in prayer.