How Can Recurrence Of DVT Be Prevented?

Hi last year after a long road trip my son at 16 years of age, suffered severe leg pains and was subsequently diagnosed with DVT on his left leg and blood test showed he has Factor 5 Leiden and his factor 8 levels were high then. Everything else was in the normal range.He was put on anti coagulation meds for 6 months.He underwent the thrombolysis procedure and had majority of the clot removed with some residual, still after six months of anti coagulation, which I understand the body will have the ability to absorb gradually. He did a Dimer test on Apr 19th and that was normal, so his hematologist said he was good to get off blood thinners. He just had another Dimer done exactly one month after on May 19 th, and his levels were slightly elevated. His doc said slightly elevated levels are still considered positive. Is this right?My son seems to be in great shape, works out at the gym everyday, drinks plent of water to keep hydrated and seems very healthy. I cannot understand why his hematologist wants to put him back on anti coagulations just because his Dimer is slightly elevated. He said his Dimer levels and his factor 8 proteins all indicate he has clot formation. His doc is involved in a study using Xarelto on pediatrics and said my son could take that even though he s not 18 yet?He also said the long road trip had nothing to do with the DVT last year? Is this true? If so, how come there is such a thing called flite tabs for people who take long haul flights and road trips?I said he was not showing any symptoms now and requested for an ultrasound and he said there was no need for that. You know, my son is only 17 now and I am taking every precaution possible to ensure he never gets another DVT so after reading and educating myself a lot, on alternative therapy I ve decided we will take that route.He takes fish oil supplements and 120 mg of Pycnogenol (60 mg twice daily). If need be I will start him on Nattokinase which seems to have great reviews from people with previous DVT experiences and majority say it s worked much better than Coumadin. Pycnogenol too seems to have done wonders for people as far as leg swelling and pain and circulation. i need to know if I am doing the right thing for my son. What are the next steps I should take. I m planning on getting another Dimer test done and if that is positive should I be concerned or is it right for me to follow up with an ultrasound?