A Trains On The Brain series story.

In the mid 2000s, twelve year old Robin lives in the commuter-belt village of Stanstead Abbotts, Hertfordshire with his eight year old friend, Timothy. The two boys have a big-brother and little-brother dynamic between them, with Robin being more mature and sensible while Timothy is more innocent and playful. Although Robin's old adversaries no longer bother him, he's still traumatized by the events of the past and he can't get over how miserable he used to feel.
The biggest fear Robin has is losing his friend, who means the world to him now. He becomes jealous of how Timothy starts to play with Jake more than him. Then, when Timothy expresses interest in taking the stabilizers off his bike after his ninth birthday, Robin resists. He doesn't want Timothy to change, he's happy with his friend exactly as he is.
Finally, Robin starts to understand himself more as he visits the psychologist over the months. Being autistic doesn't mean he's a bad person, it just means that he's different. However, he needs to embrace the changes that occur in his life, especially when it involves other people. He agrees to help Timothy to ride his bike without his training wheels, and the two friends can then embrace more adventures than they could before.