Pulmonary Embolism Support Group
By far the most common form of pulmonary embolism is a thromboembolism, which occurs when a blood clot, generally a venous thrombus, becomes dislodged from its site of formation and embolizes to the arterial blood supply of one of the lungs. Symptoms may include difficulty breathing, pain during breathing, and more rarely circulatory instability and death.
And as far as heating pads, I'm pretty sure that's impossible for a heating pad to cause a blood clot. Burns? Yes. Clots? Probably not. Genetic disorders, sedentary lifestyle, clotting after surgery, there are a bunch of reasons (as mine was from birth control pills), but heating pads... Don't think so.
I mean, here's the thing. Even if the clots disappeared five minutes after he was diagnosed, the damage they leave behind still has to heal. Some tissue was flooded with blood and oxygen and other tissue was starved of it. The heart has usually be working double-time to try to get blood past the clot. The whole body is hit hard when dealing with blood clots and you don't bounce back from that overnight.
If the pain is affecting him in very bad ways, it is a good time to check with the doc on pain meds. Some pain meds are also blood thinners (Advil and aspirin, particularly) so he shouldn't take just anything for the pain.
And, no, I've never heard of heating pads causing blood clots. I was told by my doctor to put light heat on my leg when I had a blood clot there.