Does not survive the jump and is reborn as Harry Potter with his memories of his past life.
When he meets John Watson now John Granger he recognizes him and is soon his word.
Harry must have a secret code for either or both John and Mycroft.
Mycroft must know about the magical world and harry potter and actively looking for him (for his own reasons) and then take a bigger look at the magical world and what it is doing.
John must be a squib or a Muggle-born wizard (with a muggle education and wizard education) and insist on both for harry (the schooling will be optional for you to use).