Islabikes, either a Cnoc (no gears) or Beinn (with gears).
Islabikes are expensive, but buy second hand, and you'll recover most of the cost when you sell it on in a few years.
Size depends on your son's height, probably a 20. Buy the largest one he can ride on with the saddle all the way down, and it'll grow with him and last a long time.
The weight really makes a massive difference, and helps beginners enormously.
Frog bikes are apparently equally good, and a bit cheaper, but I have no first hand experience.
Also, if your son wants to learn to cycle without stabilisers in three hours, look no further than the Pedal Project. Best £60 ever spent.