How do I know to take my son to the dr. After falling on it while skating? No bones are visable changed, but he thinks it's broken.

Get an xray. You do not have to have any visual deformity of an extremity in order to demonstrate a fracture! if a fracture is suspected then please get an xray. On a personal note: when my then 5 year old daughter fractured her elbow =nothing visible showed up on the skin--but she cried in pain all night before we took her into urgent care! --oops.
Take him to Dr. You should take him. If he thinks it is broken he absolutely should be evaluated with a good examination and a full set of xrays. Sometimes additional tests (such as mri) are needed to evaluate for hairline fractures or soft-tissue injury. If there is a nondisplaced fracture (break) and he falls again it could displace and require surgery.