Thanks Roger. He hasn't learnt to rail a board yet. What I was planning to do was go out on the Longrider myself, with him on the Go, and I'll take along a short tow rope pre-tied to my rear footstrap. So if he goes too far downwind, I can just tow him back up wind.
Not a huge range of older EVA boards around here. Probably the best chance would be a 2005 Starsurfer if I could find one. But I think the 135L version was heavier than the 2002 Go170 so that would seem to be as good as anything else I could get.