Well I have never shot one but have thought about one, the design is definitely cool with the mag storage in stock ar type opperationg system, polymer for weight savings. But I dont know if the cost is that good though, compared to maybe an entry level AR like some of the olympic arms models. If your son likes the Bushy AR then I would look that way rather than the keltec unless your getting a screaming deal.
I often wonder about the longterm ( 30,40,50, 100 years) qualities of the plastic I know the stuff doesn't biodegrade but will it lose strength or shape over time. I only say this because I like to shoot older guns (114,80, 40 or less years in my collection) and have something to pass down. I know AR's have plastic on them but it's only stocks not the frame so that can be replaced.
Just a few thoughts Totallycustom