Congratulations! It is such an exciting time. My DS was a little older when our twins came (3 and a half) but we did much the same as Kelly above. He loved 'helping' even if it was something as simple as carrying nappies up to their room or fetching me a facewasher. I just also had to be quite organised. Snacks for him prepared and in the fridge, a fresh water bottle put within his reach - because everytime without fail, I would sit down to feed the babies and "Mummy, I need a drink/food'. He also recieved a few new puzzles, dvds etc when the twins arrived which kept him entertained.
We do spend one on one time with DS but don't make a huge deal about it. If I am groccery shopping on the weekend, I will often take DS and we will go and have a hot chocolate together, or DH will take him down the footy oval/cricket nets to have a play. We often will sit down and play a board game after the girls go to bed - he loves Connect 4 and Operation. He has handled suddenly being part of a family of 5 really well!
An older sibling for twins is lovely. My girls adore DS. They follow him around and they think he is the funniest person in the world