How long would it take Lovenox to dissolve a "large continuous" clot in the leg?

I am very concerned that after 18 days your son was taken off the Lovenox by your doctor. That is way too soon and I would consider finding a new doctor since (s)he should have referred you to a hematologist. You need to see a hematologist and find out if your son has a thrombotic blood disorder especially since he formed another clot so quickly. This is going to be a major concern for you if your son plays contact sports. No blood thinner dissolves clots, they only stop or interfere with the formation of new clots. Your body and the blood flowing over the clot are what dissolve the clot. Think of running water in a stream flowing over the rocks. The longer the water flows over the rocks, the smoother the rock become. This is the same principle with blood clots.

Lovenox is usually given every day until your bleeding condition improves so the length of use before an effect is seen depends on the type of clot. In such cases it is best to get it seen by the doctor ... to confirm how much longer the patient needs to be using Lovenox.

Hope your son is totally recovered by now.