Recently I offered to allow my son to be on my auto insurance policy so he could get a car to drive. My ex-wife informed me that her insurance agent advised her that if she allowed our son to be on my insurance policy, that he then would have my less than perfect CREDIT RATING for the next seven years. Could this be true?

No, it is not true but your insurance premium is going trough the roof, I been there

No, allowing your child to be on your insurance does not effect your credit one way or the other ( unless you can't afford to pay that high price)
But remember this does place some liablity on you if he is in a accident.
And it can effect your insurance premiums for his accidents, he tickets and more.