When a child that small falls from a distance more than his height, it is very reasonable to question what the supervising adult was doing when it happened. Even a child without a skeletal immaturity would have a chance of breaking something when falling like that.

I do not know why they are saying that, especially if they are not giving him the proper medical attention: he could lose his arm if it really is broken: but you need to pursue immediately it legally if you want the issue resolved.