I always skip my middle name on purpose when issuing a ticket -my surname is very long, 20 lettters...- and my TG account is also without my middle name, never been an issue with immigrations, even the passport shows my middle name as well.

But in case your brother is collecting miles, and name on ticket and name of FF account are not identical there might be the possibility not getting the miles credited automatically (I had big issues in the past with LH, whenever my travel agent -by mistake- issued my ticket with the short version of my name, but LH FF account show the long version, I had to sent in my boarding passes after flights, which was very annoying...).