Hi, D.. Well, Honey, never underestimate the power of a new baby on the heart and mind of the older child. Just because he has adjusted to the change does not mean he is completely comfortable with it. His life is completely altered. His very identity is different. He has gone from being the baby of the family, the one and only child, to being the oldest child, the big brother. That is HUGE for a little kid! He may not know exactly who he is anymore, or what his place in the family is. Maybe you could get a children's book or two on this subject and read it together so that he can get some more information and see that it is not the end of the world. You can also explain to him what his role in the family is now, and you might have to explain it several times for him to grasp that.
Make sure he knows that you love him just as much as you ever did -- more, even, because you've loved him longer. Let him know that you and he will always have that special connection because he is your oldest child, and he will always have that special place in the family. That might be the reassurance that he needs.
There could be a number of factors in your son's sleep disturbances. Children go through a thing called Sleep Terrors, which are like bad dreams but very, very intense. Has he started anything big and new like preschool or daycare? Have there been any changes in the house or in your relationships? Is anybody close to the family ill or injured? Anything could be causing him anxiety, and even though he is talking, he probably doesn't have the vocabulary, the words, to explain it.
Maybe you could try drawing pictures with him. Both of you sit down with a box of crayons and draw the home, the family members, and so forth, and see what he does. Children his age communicate very effectively through their art.
Now, if the problem with the sleep disturbances is physical, that's a different story. Still, it can't hurt to let him know how much he is loved and needed in the family. Kids can also get hairline fractures -- you mentioned stress fractures -- and still walk around on that leg. Naturally, the pain would be worse at night because the blood and other fluids will have a chance to pool in the leg while he's lying down, and the swelling will cause more pressure -- if that's the problem. Children's Tylenol before bed might help him sleep for longer periods of time, but it will only work for about 4 - 6 hours. I hope the doctor can solve the mystery for you.
Peace,
Syl