Very tough question. On the one hand we're supposed to give them time and space, but on the other hand you do not want to appear cold and distant. Since you're under the same roof I would suggest just letting him know you're there if he needs you, so don't say things like "what's wrong" (because he'll just reply "nothing") but instead say things like "you seem a little down today, let me know if you need anything". Statements like that are good because they require no response, you can say it and go right back to what you were doing and it's up to HIM if he wants to ask for anything, and when.
