Hi, my daughter is riding the Trek MT 220. I find it to be an excellent bike for kids in this target age group (she is 9). It is extremely adjustable, from the stem to the crank positions, has seven gears via the twist/grip method, suspension fork, nice geometry.
I have no idea what it weighs, but it is heavy. She loves the bike and it's a nice introduction to off-road for her. She is learning when to shift, she is getting a feel for the fork, etc.
Bought it for $270. My son will get it when she grows out of it, bonus.
Don't remember if you mentioned it at the top, but your son, at 8, may be in that tweener stage -- between a 20" and 24" bike. If the 24" fits, go for it.
pic (not mine):
info from trek website:
http://www.trekbikes.com/us/en/bikes...es_9_12/mt220/
good luck