The fob watch containing the Tenth Doctor's Time Lord essence communicated with him and instructed him. It could do this because Latimer had low level telepathic abilities it could access. It also granted him a vision of the First World War which he used to escape a shell at a critical moment. After the Family of Blood was defeated, the Doctor let Latimer have his watch to look after and for good luck. He fought in World War I and survived. Like the Doctor, he did not like violence and refused to use a gun when the Scarecrows attacked, responding to a call of "Coward!" with "Yes sir! Every time!" an echo of the Doctor's answer to the question "Are you coward or killer?"
As a boy during his school days, Tim feared being bullied for his intellect, so he made sure to make his grades just good enough. He shared a similar belief in not fighting others, especially if they were poorly armed to combat him, just like the Doctor. He respected the Doctor thanks to what he learned of him, yet still feared the darkness within him.
Many years later, the Tenth Doctor and Martha Jones saw him again as an old man, a veteran at a Remembrance Day ceremony. By this point he was apparently wheelchair-bound. He still recognised them and silently acknowledged them, with tears in his eyes. (TV: Human Nature / The Family of Blood)
He's like fire and ice and rage. He's like the night, and the storm in the heart of the sun. He's ancient and forever... He burns at the centre of time and he can see the turn of the universe... And... he's wonderful.