my 9 yrs old brother cant do math

i have a 9yrs old brother. he's turning 3rd grade next school yr. i'm much older that him and so as my other siblings. the youngest before him is 18 years old. i learned about this problem when he spent summer at my place. we played monopoly and he doesn't seem to know what the number 100 looks like i thought that it was just probably too hard for him, so i asked him to count to 100. and by the time he reached 39 he stopped and said that he doesn't know the rest. i gave him a quiz. really simple one like 1+3 or 5+2.. when he's finished i checked his answers and got a score of 2/10.. i got really worried coz for his age he should know how to do addition.. what scared me the most is his answer at 1+1 is wrong.. =( i tried to teach him.. but he wont listen.. he would make a face like he is about to cry or will look really depressed. i figured that this is probably what he does at home to they let him go.. i tried to make him watch cartoons that are educational.. but he doesn't like it.. he likes spongebob and some computer games that are not educational for his age.. i told my mom about it but she denied everything she said that he's just probably shy to answer the questions etc.. but i know that he didn't know it.. and i don't know what else to do.. i want to know what kind of teaching techniques i can do for him.. the kind that he will get really interested and have fun.. i dont want him to grow up dumb or get teased at school..

[LosingMyMindInGA]

I don't know about the school district where he goes but where my kids go they have to meet certain standards in all subjects in order to be promoted to the next grade level.  If he is continuing to be passed more than likely he is doing well enough in math at school.  But for whatever reason he's not wanting to do it outside of school.  If you are really concerned, try discussing it with his teacher.  He/She may not talk to you being as you are not a parent or gaurdian but if they know you and know you are his sibling they may be willing, especially if they have tried to talk to your parents about any potential problems without getting a receptive response.  I think the best place to start is with the school to see if he is indead having problems.  If he's doing his school work well then there isn't much cause for concern, he could just be playing games with you or just doesn't like math and doesn't want to do it outside of school.  If a problem exists his teacher would more than likely have picked up on it by now and be able to make suggestions to help him improve.