OK, everyone. Another slightly <1/4 turn on the rod has got the bow right and it's held for 36 hrs now. A double layer of the masking tape has the neck angle right. I've run through the setup (I've done this sort of thing before but isn't Carruther's Yootoob 4-parter wonderful?).
The upshot is, it's pretty well set up and light years ahead of how I got it for playability. The action is still a bit high, because I haven't filed the nut (see later) and it's definitely not right for 45-100. Also, it still takes quite a strong clamp to bring the strings down to the 'board. OK for me, but my son's pretty much a beginner.
I'm going to encourage him to play it more and take it from there. My crystal ball reckons a set of 40-95s will be required; reducing the tension and sitting in the nut better for a lower action. In which case, I'll go with Bobster's recomm for the D'Ad chromes. They're also rather reasonably priced.
Many, many thanks for all the input I've received from everyone. You've not only improved the bass, but at the cost of a few inches of masking tape and a future restring