How to get my lazy brother to stop being "sick" all the time?

Okay, my younger brother is about 13 years old, he always seems to have some kind of “ill” feeling when he has to go to Kung fu. He hasn’t been there for awhile, and my sister and I both know that it’s because he’s lazy.

He’s able to go to his friend’s house and do whatever, but after he comes home, he suddenly feels “sick” with a “horrible feeling in [his] stomach”.

Or, when I am about to go work out somewhere, he’ll want to tag along, I’m fine with that—but then after, when he has to do some sort of homework, or go to school, he is now suddenly too “tired” or has a “sore throat” and cannot go.

There are more examples, but those are the ones I could think of that just happened in the past two days.

Anytime there’s something he wants to do, he’ll go even if he feels ill, but when he’s lazy, and doesn’t want to attend his Kung fu class, or school, there’ll be an excuse and he’ll just stay home playing rpg and online games. =|

Is there a way to stop his laziness? How would you go about doing this?