Great to hear your uni background, Ben!
And to the OP:
+1 on unicycle.com - just pick the appropriate store for your country (it’s an easy option on the website). For more info, talking to them on the phone may also be useful. If you’re set on shopping other sites, a Torker LX (but not the CX) is also a pretty good learner uni.
For a 10yo, a 20" unicycle is almost certain to be the best size. You should be prepared to cut the seatpost down to make the uni fit for him. This can be done with a hacksaw, or better with a pipe cutter ($10-15 tool you can get at the local hardware store). If you have any thought of trying to learn yourself, you might consider getting a second seatpost from unicycle.com.
I started the uni trend in my family about two years ago, and now have my 10yo riding trails with me, and my 12 and 9 yo’s are learning. We now have six unicycles ranging from 16"-36".
But if nobody in your family rides, I understand the whims of a 10yo can shift quickly, so going the route of a less expensive learner first may make sense. But you should also learn to ride, as it’s a really good activity to do with your son! Check out my video of riding with my 10yo: Hiawatha rail trail with my son