Hi D.,
I have a 25 month old toddler who shares a room with her 8 year old and 6 year old sisters. I understand where you are coming from. She doesn't get up as much as your son but we have had some rough nights. What I have done that works sometimes is to scoop her up when she gets out of bed and put her back into her bed without talking to her. I've also had to be very firm - the "mommy monster" is how I describe how I am after midnight. Not scary just REALLY needing my sleep. He may wake his siblings up but some crying is inevitable. Could you close their door and put a noise machine or fan in their room? I've also found that she wakes more when she doesn't eat a good dinner. A bedtime snack of graham crackers helps.
My oldest had a hard time staying in her bed when she was two. She was very verbal and understood sticker charts, etc. We had a reward system set up for x amount of nights in your bed without waking mommy equals a treat of some kind. I hung a calendar in her room and used star stickers. I don't think this would work for her sister right now but maybe for your son?
With my youngest I've also started talking to her at night about not waking mommy up, stay in your bed until daylight, etc.
Sorry if I've rambled....need more coffee....two year old woke me up at 5:20am but is back to sleep :)
J.