Glad he's doing well. That's great.
Everyone has made great suggestions. The museli bars/snack food are a great idea for mum. When you're not at home and going back & forth from the hospital finding time to eat properly is a challenge. It's great that she's at Ronald MacDonald house, but I don't know what happens if she's up to express at 2am and is hungry ... snacks in her bedside drawer at that point are invaluable.
Offering to help out with her other ds and around their home will help relieve them of stress too. And she'll need extra trips to doctors/etc when bubs comes home, so she'll probably need you more then too.
Baby Bunting have some great prem clothes - depending on the store they don't have a huge range, but they are really cute & practical (open flat, holes for tubes/etc). Otherwise, as already mentioned try Mothercare (although I'd buy in store rather than online as I think their online store has had issues) - their prem range is great! I know when my ds was in special care he needed temperature checks at least every 3 hours (under his arm pit) and so the nurses were quite fussy about what type of clothes he was 'allowed' to wear ... although most of the time he was just in a nappy.