race setup for 125sx Help

My 17 year old son's first moto practice was today, and the problem is he cant get enough rear traction, the bike is a 2003 KTM 125sx all stock with new michelin starcross tires, with me on the bike it plants and goes but i weight 200lbs, son weighs 140b should i loosen up the rear mono spring? it was really muddy today which seemed to make it even worse.  

This sounds like a rear shock set up problem.  Here is a video on how to set the sag on a KTM http://motocross.transworld.net/2008/02/05/tuesday-tip-setting-sag-on-a-ktm/.  He may also need a lighter shock spring to get the best action out of the shock.

Here are some adjustment tips for the compression and rebound disregard any sag setup procedures the KTM is different than the Showa/KYB but effects of sag, balance, compression and rebound adjustments are similar:  http://www.tootechracing.com/Twin%20Chamber%20Setup%2010-12-07.pdf  
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Colorado

2006 RM-250

Thanks for that tip Mark.  I just had the suspension on my XC-W stiffened up a bit and had new Dunlop MX 51's put on.  I tried riding it on Tuesday and something felt really, really wrong.  I figured it was because of the new setup/tires, but after 20 miles, something still didn't feel right so I brought it back to the mechanic to look at - I figured better safe than sorry.

It was ok on the straightaways, but as soon as I went to go around a corner, the whole front end felt wobbly and the rear tire kept washing out.  This is my second XC-W so I'm fairly familiar with how they handle.   After reading that article, maybe it's the setup that's wrong.  
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Hudson Valley NY
KTM XC-W 250

"If you don't have a dream, how can it come true?"

Thanks for the input, I adjusted the sag to the max and it helped a bit, I really think at only 140lbs the rear spring is still a little too much, anyway he got through his moto with only a drop, oh and a gate error, finished 18th out of 25, one thing i thought was weird was he was racing the d lites beginner class but was racing against experienced racers that moved up to the 250's from the 85 expert class so you can imagine.