There is no set amount of time that an insurer can be prevented from sending a person for an independent medical exam because each case is different. Usually, a reasonable amount of time must pass before an insurer can send a person for another exam. A change in condition, such as a potential recovery from surgery may also allow a new exam.
It is usually a warning that a cut off of benefits is around the corner. If you don't have an attorney, get one ready to fight when they cut off benefits. Sad state of affairs here in Michigan.
My answer to you question does not constitute legal advice. Only an in person or telephone consultation will result in an attorney/client relationship. Please do not rely on brief answers without checking with an attorney in a confidential consultation.
It is true often times it is a precursor to trying to cut off benefits. In your case they may be sending him to different doctors for each condition. If you get cut off consult an attorney ASAP. Good Luck!
A roundup of the best tips and legal advice.