I efiled my son’s return using a different online service. I mistakenly had him shown as not being claimed as a dependent on another return. When filing using TT, my return was rejected due to me

If your son's e-filed return has been accepted, then you cannot e-file if you claim him as a dependent.

You have to file your return by mail, Your son has to amend his return to indicate that he can be claimed as a dependent by someone else.