typicaly the kx 65 comes with a 4.9 shock spring and a .27 fork springs and are a little stiff for your application..
For your sons weight I would use a 4.8 shock spring and .24 fork springs.
Did you buy the bike new ?
Also what year is it ? If it was a used bike a good fork and shock oil change could also help dramaticaly.
If you want a top of the line fork -- MX-Tech makes a drop in fork kit that drasticaly improves the fork and brings this older fork design up to modern day .
The kit is called the "Betty65 cartridge kit"
you can check out some info at
http://www.heavydutyracing.com/mxtechsusp.html