Blood clots

I am a mother of 3 sons. 2 of them had blood clots in their 20's. My oldest is 35 yrs old and 7yrs ago had a "clot" in his brain. The medical emergency controlled it with Heparin and he was released without any further medications. He was told that this has a very very slim risk that it may happened again in his life. While in Sept of 2008, he had a DVT. The Doctors prescribed Coumadin 10mg/life.

My youngest son who is 26 yrs had a DVT last year in Sept. After taking Coumadin 10mg until Christmas, the Doctor told him to come off and go through some Gene tests.
The MTHFR came abnormal with Protein C function at 0.62 (Should read above 0.70).
While Homocysteine level is at 8 (between 5 to 12).
The Hematologist is NOT concerned at all and suggested for my youngest son NOT to take Coumadin and repeat the test along with the parents.

While both parents, we took the test recently and I (mother) is normal and my husband (father) is low normal. My (youngest) son's came at the border line.

While my oldest son had his test done in April 2010 (while on Coumadin) came as Heterozygous detected and MTHFR mutation A1298C.
I thought the gene test can't be taken if one is taking Coumadin?

What can be done in this case? Are the Doctors right by prescribing my oldest son to stay on Coumadin and my youngest not to?

What do you think about prevention on Nattokinase?