This is so normal for a kid.Take it easy.It is good for the kid that he could not remember his sick episodes.Why do you expect him you to remember those episodes of vomiting?
You should be worried if his calculating skills of higher functions math got deteriorated or his Minimal Mental State Examination scores are way too less after that events of being sick.If he could not remember the alphabet or could not recite The Children Rhymes or could not properly do his home work,you should be worried.
My concern is because he normally doesn't forget anything! My reasoning for putting his intelligence in the comment was basically to show he forgets nothing! The episode was only 2 weeks ago and he not only doesn't remember the vomiting ( which I realize is a good thing). He doesn't remember the entire day. Since he was sick I got out the portable DVD player and laid in bed with him and watched movies with him and he doesn't recall watching any of the movies or even using the DVD player.
It is highly likely with him being so ill and possibly his temperature was high, that he wasn't really with it.
Now that he has recovered from his bout of sickness and is fine then do not worry. If the memory problem continues then do get him checked out. But do keep in mind that as children grow that can become selective in what they want to remember or what they want to talk about.
No that he is better, take the DVD and television and any other electronic games away from his room.
As you know that he is very bright and can work things out (a lot of adults have problems on the computer and ordering on line); keep all your cards and keys out of his reach and set rules and boundaries.
Some children will be very good and not do things that they shouldn't, others are more daring and will try anything when out of sight. You know your child best.
Best of luck.