Based on your likes/dislikes, I'd head to Mabie, or perhaps Dalbeattie… but avoide Ae.
I really like Ae, but it's very fast and jumpy feeling place. no much in the way of natural rooty stuff. There certainly are some rocks… but I'd say it stays too much to the jumpy side of things for you.
Dalbeattie is much more flowey single track – though it's very pedally – not much in the way of freewheeling descending. Definitely a good work out. There is some nice natural rooty sort of sections, but may be a bit much work for your boy, dunno
Mabie – highly under-rated in my book… think it's great. flows well. Couple of nice climbs, nice fast flowy singletrack descents, lots of rooty stuff, bit of rocky stuff, nothing too technical. Talking about it is making me want to go there.
oh – and if you like natural technical rooty trails – you should definitely check out Drunlanrig castle, if you haven't all ready – though I doubt it'll be open boxing day.
Enjoy your ride, wherever you choose.