Did Holmes actually remember Umezaki Senior, or was he just being nice?

At the end of the movie he tells the young boy that he has just finished his first work of fiction, the letter.

His letter was a work of fiction. He did it to make the Japanese son feel much better about his father.

It's the moral of the film. Sometimes it is better to be diplomatic and sensitive to people's feelings than to be coldly logical and oblivious.

He felt that if he had been kinder to the young wife she would not have committed suicide by walking in front of the train. He did not wish to make the same mistake with his Japanese friend.

Of course he was telling a fib to the son to make him feel better. He learned from his mistake with the woman that sometimes it's better to tell a white lie than to be harsh as he had been with her. What harm would it do for him to tell the son a story of his father's bravery and service to the country? It would make him feel better than to think his father had abandoned him. This was the whole purpose of the movie- Holmes coming to terms with his mistakes in the past.

The "some British official" Holmes is with is his brother Mycroft. Mycroft is essentially the British government in one man at the time of the stories. If he wanted a man, the man would have to be valuable. This helps to understand why Holmes would be so cavalier in telling the man to abandon his family. He is not a particularly nice or family-oriented man in the stories.

I think he made it up to make Umezaki feel better.

During the writing of the letter, Holmes tries to come up with a name for a location to include in the letter, he sees one of his book's title and "steals" its name to conjure a fictional place so the japanese son would think his father went to somewhere really important. Also at the end, Holmes tells the boy he just finished his first and only piece of fiction.

So the entire letter was a lie. The father ran away from his family and Holmes never met him. But as he said at the end, no one should die without a sense of completion. So he sends the letter so the japanese son can live thinking his father was a honorable man.

English isn't my primary language, but at least I know more words than you

But surely he then told the son his father was a Traitor to his own country. At the time this is set the memory of that war will be fresh. Yes the father may well have been taken on pre WW1 when Japan was an ally of sorts.

Seems pretty simple to me. Homes laid out the facts of the case to Umezaki in his original meeting with him. He never met the man's father. The father wrote up some letter to abandon the family. Later on when talking to the mother of the boy Holmes realizes that he should have lied. So Holmes fabricated a meeting with Mycroft and The father. This meeting never took place. Holmes just wanted to make Umezaki feel better.