No its not over reacting, has he been watching any programmes on TV that may affect him emotionallyanything new happened in his life ,, I think you hit the nail on the head when you said he had some normal jealousy feelings, you could always ask him ,say something like' this is lovely Johnny Mom is so glad that you want to hug her'see if you get a clue. Perhaps give him lots of hugs back,and accept it whilst it lasts ..,