eCredits cannot be used with Pay with Miles itineraries.
You will have to book two different PRNs. If your son is younger than 15 this is impractical since he will be considered an unaccompanied minor.
he wouldn't be an UM if Dad flies on same flight. You don't have to be in same reservation.
Also note that if the ecredit can only be applied to OP, then can't do a PNR with both. Whatever methods of payment are applied, get split half & half for each passenger. Thus if the ecredit is only good for Dad, can't book a 2 passenger PNR with son.