I'm going to add a little to what a few others have said.
First of all, most pediatric dentists do not want parents around during a cleaning and exam. The kids have a tendancy to act up even more, and I highly doubt that the assistants or the dentist would do something to hurt your son. (For the record, I am not a dentist nor do I work for one.) When mom or dad are not hovering around and answering all of the questions for the kids, the dentist can get his/her job done faster. With parents in the exam room, the stress factor goes up exponentially for children and for the parents. If you feel that you need to find a new dentist, fine. Ask for referrals from other friends and neighbors with toddlers.
Next take a deep breath and wait to see what happens with the x-ray. A cyst-like object can start to form days after any fracture, so it seems likely that this would not have been caused at the dentist's office. If he does have a fracture, who knows where and when the rib broke? He could have fractured it while playing, being rough and tumble, or almost any other circumstance. Believe it or not, kids' bones can break somewhat easily because they are more flexible. So wait to see what the x-ray shows and relax. Focus on your son and your family.
Lastly, and I'm going to be a bit mean about this, you don't need a lawyer. You don't even need to be thinking about a lawyer. This thought process leads to nothing but trouble, and your husband is right. You cannot prove anything either way. Let it go and help your son heal.