M.P.
My suggestion is to talk with the child. There are people who will tell you that children do not know why they are sad or angry etc. but it has been my experience that many children do have some ideas about thier feelings. I would tell him that I am worried about him and really want to help feel better. He may have some answers and then you can move from there. I always ask children what I can do to help. Tell me what I can do to help you feel better.As younger siblings grow there may be some jealousy going on or the fact that he is away from you all day may be causing him grief. I would not suggest what I thnk it is but let him try to tell you.