I have a Kelty Meadow child backpack carrier that we no longer need. It is in good condition and works well. Great for hikes with the little one who can't ride yet. These are going for about $50 to $75 used on ebay. Selling for $40. Looks like this:
He has actually slowed down on the growth a lot, used to be in the 105-110 percentile for height and weight. It is the hardcore powerlifting regimen I have him on combined with nightly meals of steak and potatoes.