do or redo your research. The Sr Adv is drum-braked, air-cooled play bike. Doesn't mean it couldn't be 'raced' but it's competitiveness will depend on it's condition, your son's ability/desire/experience and that of who else shows up.
download, read, understand and do what the the 50cc owner's bible suggests.
http://www.users.bigpond.com/steve.m...-ktm-bible.pdf
sounds to me like the clutch is not set up properly, worn, using the wrong oil.....have you even been into the clutch yet? what springs/stack combo are you running? oil type? amount?
The 3rd happiest day of my life was the day my oldest son moved up to a KX65. Now I am going through the whole 50 thing again with my youngest.