Neither of my children have arm joints like your little one but it does sound as if your baby might be double jointed. Have you been to see a doctor? My wrists, ankles and knees creak and crack when I turn them and my hips can be bop out of place ( not that I ever do this ) but the doctors have all said my bones are stronger than average but they do get sore quite easily. If your baby isnt crying when his arm do go back then I really believe he is double jointed but do get it checked out. Hope everything turns out ok and let us knoe if it does!