Is my second grandson following in his big brothers footsteps??

Hi My names Kev and this is my first post on this excellent site. i have 3 grandsons from my eldest daughter. Alan aged 12 was diagnosed as autistic when he was 5. challenges apart he is a lovely boy, for the most part. his younger brother aged 6 is doing well at school and appears not to be following in his brothers footsteps. sadly the youngest aged 2 does seem to be following. he can count to ten and knows all his colours but has never uttered any other words not even mum & dad. His mum who has been so fantastic in handling the boys and encouraging everything that they wish to do does not seem to think that there is a problem with the little one. should we be worried. Kev