Manual for XXX KE is here:
http://www.losi.com/ProdInfo/Files/losa0029-manual.pdf
it's pretty much the same build as standard XXX manual, there may be a few differences with part numbers but give me a shout if you get stuck.
couldn't find an original XXX manual but chances are it's well hidden in archives as it's a fairly old kit.
Noticed yours has the UJ driveshafts which were a later item on XXX so must have been retro fitted, possibly from XXX CR
noticed that you have the axle spacers behind rear wheel nuts for some odd reason... possibly done because the driveshafts are from a different model... they should sit between rear hub bearing and the drive pin.
I's American so it's imperial hardware/tools required.
Good stuff starting him off at 3, could be a future champ in the making and all for encouraging youngsters to take up the hobby... fear for your wallet though!
the XXX is a fairly strong beast so hope it does him well.