My son is 4 months old and he sleeps through the night, but he sleeps in his rock n play. I'm worried that he's starting to get too big for it, plus he is getting a bit of a flat spot on the back of his head, so I want to transition him out of it. I tried having him sleep in the crib, but he only slept for half an hour and then started squirming and fussing. What can I do?