Islabike CNOC 16" (or he might be tall enough for the Beinn Small 20", which has a much lower standover than most of the 20" bikes)
The CNOC 16" is 13.2 pounds
A friend recently got one of these for her 5 year old son who is also on the small side and he just flies on it.
Here's her blog posting about it:
Santa is bringing my daughter (who is 5 years old with an 19.5 inch inseam) the Beinn 20 small for Christmas. It is a gorgeous bike. The gear shifting is smooth, the brake levers have a light touch. It weighs 17.3 pounds, which is about 2 pounds LESS than her current 16" Hot Rock.
My second choice would be a Specialized Hot Rock 16"
We had the Novara 12" Stinger and that sucker is a brick. We also had a Trek Mystic 16" (the girl's version of the Jet) and it was also very heavy and has, in my opinion, awkward geometry. The handlebars sit up very high, which puts the center of mass very high and makes it harder to handle. My daughter learned to "ride" without traning wheels on the 12" Stinger, but it was basically to ride short distances or ride around in circles. She outgrew it but wasn't able to handle the 16" Mystic without difficulty, which discouraged her from riding. It wasn't until we got a 16" Hot Rock that she was really able to ride. Now she regularly rides up to 4 miles on her bike.