It really is time for ds (8.5, but around size of average 11 yo) to learn to ride a bike. The problem is that all kids his size are already adept & so bikes big enough for him either (i) come with 18 trillion gears & weighing a ton (extra-difficult to manage when you have balance problems to start with) - or (ii) are racing models made of carbon & just not sensible.
So - where should we look for a totally simple, 3-gears-at-most (needn't have any at all, really), light, ordinary, bike?