Should have said I'm 51 and my son 10. We won't be doing any jumping,just trail riding. I've lost a lot of weight and am getting in a lot better shape wanting to get a lot better and he needs to get off the couch with dad for another activity that he might really like.
Probably the best reason of all for your weight loss is doing activities with your boy. He will cherish the memories and you'll be promoting an active lifestyle for him.
16bore is right. He will outgrow his bike quickly so it doesn't make sense to buy top of the line unless you find a smoking deal on one for him.
If you go used, check the gear teeth and chain for wear. Look over new ones as a point of reference. Trail grit will grind them down. Use a dry wax based lubricant designed for chains and gears. Look for bent or damaged spokes and wheel wobble ... easy to check. Gear changes shouldn't take forever.
Suspensions ... they make a ride smoother, but they also absorb energy when going up a steep hill as they will flex some.
