Since I have been negligent in welcoming new members/builders of late, I'll try to improve my batting average by saying "Welcome" and saluting you for introducing your son to the world of DIY via a pedal circuit. I just hope you won't mind if he starts wearing all black and eye-liner, now that he has fuzz.
As PRR/Paul notes, using a pair of LEDs will be both louder and somewhat less distorted. Insomuch as that may be what you're going for, it's not an "error" as much as a choice. However, if you wouldn't mind getting a little more grind from the circuit, using a pair of simple silicon diodes, like 1N914, 1N4148, or 1N4001, etc., will strike a balance between sizzle and output level. Not that loud is bad, but when a pedal is too loud, the range of usable volume-control settings becomes restricted and "twitchy".
Note that the original was in a tiny box that plugged directly into the guitar, (as the Dan Armstrong boxes did, though some had the plug going into the amp, as below) leaving no room for a Gain/Drive control, so the gain was fixed. I doubt you'll be boxing it up in similar fashion, so if you'd like to be able to get harder and softer overdrive, I suggest you make the gain of the first op-amp variable. You could do that be either replacing R3 (470k) with a 22k resistor and 500k pot (wired as variable resistor) in series, OR by inserting a 50k pot (also wired as variable resistor) in series with R2 (1K). Either way will let you vary the gain from mild overdrive to max.
