If he gets out of hand, print this >> https://www.thingiverse.com/thing:3087440

I made a new version that fixes the problem with the joints wearing out quickly. I redesigned the sockets so they are more flexible and should maintain a snug fit.
http://www.thingiverse.com/thing:1912000

This was inpired by http://www.thingiverse.com/thing:1067086
This is an excellent design, but had some issues.
The biggest problem I found in the comments was the weakness of the legs.
The best way to fix this is to print them laying down, but the way they are designed, they cannot be printed this way without a lot of supports.
I decided to re-create this from scratch with OpenScad.
I used DexterMurphy's design as a template for sizing, etc.
I used this skull for the abdomen (heavily modified with MeshMixer and OpenSCAD) http://www.thingiverse.com/thing:36666
I tried to make this with minimal supports (the supports required are already in the .stl files).
I have printed this in ABS and PLA and both work great without adjustments.
If printed in PLA, you don't need rafts.
I included an ABS version with custom rafts (easier to remove) if you have a printer without a heated print plate.
I don't have a heated print plate, so I can't tell you how it would print with one (without rafts).
If you are printing the rafts version of the legs, they are already laid out for you.
If you are printing the non-rafts version, you need to print the following:

4 - LegTipLong.stl (front and back)
4 - LegTipShort.stl (middle 2)
2 - LegMedium.stl (back)
6 - LegLong.stl (front and middle 2)
8 - LegShort.stl (attach to the body)
2 - Fang.stl
1 - Head.stl
1 - Abdomen.stl -or- AbdomenFangs.stl

I recommend printing this in ABS (or some other soft plastic). I gave the one I printed in PLA to my daughter. The joints seem stiffer at first and wear out very quickly, becoming loose and sloppy. The ABS one moves more smoothly and doesn't seem to wear out as fast.
Update
Fixed in new version here: http://www.thingiverse.com/thing:1912000