I think children develop a sense of ego very early on. My son didn't want to wear 'baby' clothes as early as age 3. He would refuse to wear one piece pajamas, preferring the two piece look.
Perhaps this kid just doesn't like adversity and he has a low threshold for it. It's no different than my son who doesn't like to practice writing his letters because he's not good at it yet and so he will make sometimes very imaginative excuses to try to get out of it.
I'd say that wasn't ego but independance, something that should be encouraged.
Children want to please the adults they love so will of course want only to do the things they are good at to gain the praise of those adults. If they are praised for the trying and the effort they put in rather than praised for the result they will learn that if they try hard they will still be rewarded. The end result is not always the desired object instead the desired result is in the
trying of something new and hard.
I agree with Bluekey's very good post. It's a case of knowing the child and treating him as a child not as a small adult. Sometimes something as simple as a sticky badge saying 'I tried hard' or 'Good effort' is enough. I always find something to praise while correcting, even with the adults. Another thing both childen and adults fear is being laughed at if they do something wrong. I jump down so quickly on people who laugh at others mistake I can quite scare myself lol! Fear of making a fool of yourself is something to take into account when teaching, the students fear and your own (it crosses my mind all the time)
My daughter does cheerleading ( I thought it was a bit girly until I saw it, scared me silly as she's the 'flyer', she does do MMA as well though) anyway this afternoon they had a 1st Birthday show with everyone taking part. Much of the children's routines I could see the same small things that I see when teaching children kata but one thing did impress me. One small girl got up to do a routine which she was doing very well until she forget what was coming next and got lost. She burst into tears and the coach comforted her but what the others did, being cheerleaders lol, was break out into a chant, it went 'We are so proud of you, we're so proud of you', you'll know how it went. It gave that little girl a huge boost to know she had done her best and while it hadn't gone according to plan everyone appreciated her efforts and they were proud of her for trying. Now I know we can't start chanting in martial arts lol but fostering an attitude in class like that can be done.
You can't say well done for something that isn't well done, the child will know it's false but a well done for trying, for a good kick or for even knowing they made a mistake and redoing it, is good. Tell them you know it's difficult, that you found it difficult when you were learning it and telling them you know they will try their best often works too.
Be inventive when teaching. I had a lad who had trouble with his turns in kata he could never get the right direction even when told left or right so I gave him one of his socks to hold in his left hand and a very light little weight in the right and shouted heavy hand, light hand instead. It made all the difference as it was a physical thing he could connect to rather than left and right which is a mental thing.