Many of us love the adventure stories of our childhood such as Enid Blyton's Famous Five, although children today are no longer allowed such freedom.
These chapters bring the sense of adventure into the present day by making Tristan a modern character who finds his own adventure near his home.
The addition of Sebastian gives this tale a modern slant and a further level of depth as it relates to people being different and the way in which those differences can be understood and overcome by mutual respect.

There is good suspension in this story, especially as Tristan is very often the only character on the page and this is well sustained. Perhaps as the story progresses and we see more interaction between the characters the dialogue will become stronger.
A further title and/or subtitle would help young readers ascertain what genre the story falls into.
Good writing style, research ideas.
A good read so far.