We think my little brother MAY have dyslexia?

There's four of us. My sister, 23, my older brother, 17, me, 13 and my little brother who recently turned 7. We have all been really shy growing up, but my little brother is especially shy. Teachers even told our mum that there worried about him. So, when my mum went to parents evening teachers told her he was extremely behind. He has already, once been tested for dyslexia but doctors said he was fine. A year then ped and he is now nearly going into primary school. Teachers have started getting worried about him again though, he isn't a trouble maker and really does try in school. He can't concentrate very well though, he gets bored and will move on to something else. Its quite weird though, his grandfather will tell him all these wonderful things, about countries, spaceships, planes and weeks will go by and he still remembers these things, but then at school, he can't even remember full stops, capital letters, he can't even read his own writing. He already attends sessions with teachers, with three over pupils from his cl that are very very shy like him. At first, we all did actually think his shyness was to fault, and that's why he doesn't really know these different things, but yesterday i was talking to him and i asked him where all the different things go in sentences, he knows where and what there for he just can't do it. In his cl, they've been doing full stops and capital letters for almost 6months now and he still can't do them. My mother said to me, i actually felt like crying when i saw his work, how different it was to the other kids, she thinks she's to blame. I just remembered too, when the cl do there weekly spelling test, he's always getting 7, 8, 9 out of 10, so why do they keep saying they think he has dyslexia? Do you think we should get him tested again? or is he just a slow learner? thankyou in advance! xx