DSD is an awesome sleeper.
DSS is shocking. What I have noticed to be "wrong" with him is that he 'wakes' between sleep periods (maybe every 2 hours ish) and sits up automatically. Looks around, plays, etc. He absolutely refuses to just go back to sleep, even though it is dark (and therefore sleep time).
Yet, he falls asleep on the bus, train, car, sitting on the couch, etc all day long.
We have tried everything, but it doesn't change. For now, we have told him that he not allowed to leave his room (other than to use the loo) before we get up in the morning (6.30-7am), or he will not get his own room when we move (DSD will- technically they will have their own rooms, but will sleep in "DSS's room" and DSD has every right to not let him in her room as "his" room is not his).
So far, we have had a whole week without him coming in to wake us (or waking DSD)*, though he still wakes up.
*This was the problem- him waking DSD "just because" (his words), or us... not exactly his waking. Though that is annoying as it means we cannot do anything with the kids at the weekend because he is exhausted, and when exhausted he whinges and cries at everything (even stuff he loves).