Well for me your choices are the western Stanes and Drumlanrig, i.e. Mabie, Dalbeattie, Ae, Kirroughtree and Drumlanrig. There is also the little Glentrool loop which never gets a mention, but that is more a loop for youngsters and beginners, and the huge Glentrool loop is maybe an all day 7 or 8 hour epic.
I normally only ever ride one a day.
For me Kirroughtree is just fabulous, and also Drumlanrig too. From the other three, try and get some locals to show you the off piste Dalbeattie, as that can make for a bigger day, although you could combine Dalbeattie and Mabie in a day if you have the legs. That just leaves Ae, and you can combine the red loop with the Shredder, a red level DH track but which is good fun on an HT so long as you dont go for the huge gap jump 🙂 If your son loves a little bit of air time then go for Ae. Mabie is fabulous, but again may warrant some locals showing you a few off piste bits, and it also has the jump park if that is yours or your sons thing.
Either way, you will have an awesome time. Of course you could make the trip over to either Glentress or Innerleithen, but they are fairly slow drives, so factor in the time needed to get there on narrow country roads. If this was me I would prefer to stick to just the Western Stanes and then do the other two som other time.
I know there is also Newcastleton too, but that is not really worth going out of your way for. It is more a place to combine with Kielder.