Sorry to hear about your brothers passing. Enjoy getting to know your little man during this hard time as much as you can.

I like the scrapbook idea but do it from when he was born.
I know kids love knowing about their parents as kids. Get her mum to write how they met and how she was proposed to and also their wedding day.
If your parents and other members can also write down some things they remember about growing up with him.

Writing down thoughts is a great way of remembering a loved one and it also helps the hurt a little. Remembering the fun times is just a important as the down times.

Other times that they will need support is the first anniversary of things, birthdays, wedding anniversaries, fathers/mothers day etc.