My son injured his foot but Dr. says he is fine?

Yesterday my son injured his left foot. From my understanding (I was not home when the injury occurred) my son bent his knees to sit on the floor (looked like he was going to sit on his knees), when his knees touched the floor he started crying that his foot hurt. When I got home, I was told what happened and I looked at it. It wasn't bruised on the outside but it was swollen. I elevated the foot and put ice on his. I thought he has sprained it and figured he would be fine. About 5 hours later, he still could not walk on his foot. I took him to the hospital where they looked at his foot and did xrays. The doctor said that his foot was not swollen (but it was because I could see that it was significantly bigger than the other foot.) and his xrays were normal. They sent him home and told me to give it 2 days and if he still couldn't walk, I was to take him to see our family Doctor. Today he still cannot walk on his foot. His this just a sprain or could something really be wrong that the doctor is not seeing on the xray?