You're going to have to have the whole fare (for yourself) in M, even if you're just using the SWU one way - can't mix the fare to the best of my knowledge -

I'm afraid that as a minor, even as one who will not need the UMNR service, you'll have to call in (and since you CAN'T do that online, we should be able to waive the DTC) and we can book your son on the same itn on a different PNR and in the less expensive fare class.