Ok, so if you have a blood clot fear don't read this!!!

I normally am ok about blood clots, for some reason don't worry about it. But on the news today they said Nick Cannon developed blood clots in his lungs due to being on flights a lot. I am not worried for me, but rather for my boyfriend, who flew every weekend this past fall and winter for work!! Most flights were about 3ish hours in length, but still makes me nervous!!! If he had blood clots, would he know by now? Or what if they are attached somewhere and haven't broken free yet?