It sounds to me as though you currently train in Tae Kwon Do, and your oponent also trains in Tae Kwon Do?
I have to assume that is correct. I have sparred many opponents that did this move to me. I know how frustrating it can be. My own son (doesn't listen to me) has this happen on a regular basis.
There are a cpl ways to deal with an opponent such as this. I don't know your tournament system or rules though. So I don't know what is allowed. Please take all of my advise with a grain of salt and pick whatever
works for you.
1) He is kicking with the right leg with a Mawashi geri (round kick). Step in with your left foot, push block at his KNEE with your right hand. If he is kicking to your middle, then punch over your right arm at his face. If he is kicking high, punch UNDER your right arm with your left hand at his solar plexus or stomach.
2) Go to his backside. His left leg is on the ground, and his right leg is in the air. Be faster than he is. It really isn't that hard. Once he chambers that leg, move to his backside (your left in this scenereo) and punch his head or kidneys.
3) Take the warning. If someone is doing something to me that I am having a problem getting around, I will on occasion do something that will earn me a warning from the ref. A sweep for example. If a tigertail sweep is not allowed, but you're really good at it, then do it. The ref won't give you a point, and will most likely tell you not to do it again, but...Your oponent will be less likely to allow himself to be put into that position again. Especially, if he gets the wind knocked out of him in the process.
4) Play YOUR game, not his. Don't worry about what he is doing as much as you are concerned with what you are doing. Free your mind, and don't obsess on what he is doing. If all you can think of in the match is "Oh great, there he goes. His leg is chambered. I've lost" Then you can't think of ways around it OR what to do if he doesn't utilize it. Free your mind and think of the present, not of the past. you've already done the past and you can't change it. You can LEARN form it, but you can't change it. You can't think of the future, because it isn't here yet. You can plan for it, but it is not a certainty. Think about what is going on in the present. HE goes to lift the leg, jamb it before he gets it chambered with one hand while you punch him with the other.