I know he is young but try getting a grass hay like coastal or Bermuda and it it up finely with scissors and soak it in water with the Bermuda for 5 minutes then mix it well then serve
Could be a bunch of different factors. Could be temperature or humidity could also be from whoever raised the tortoise before you got it. Also could be the tortoise Genes or where tortoise came from all tortoises grow at different rates. I have one that is almost 8 lbs that is about 2 years old...