three things to do.
turn the fuel off at the tank .
1) then drain the fuel out of the 'carb' ( i'm not getting into a spelling battle. ) there's a small drain screw on the bottom of the carb .
2) drain tank then remove the tap assy from it. check the filters parts are clean/clear. and the small bowl on the tap (semi sphere with square nut on bottom of tap. ) is empty of debris.
turn the fuel back on after!!!
3) as tourman said; soot in exhaust. the baffle in the rear part ( silencer ) of the exhaust system should be removable. take it out and clean off the oilly sooty deposits.
if no difference then it could be inside the carb, technical job you need a fairly good level of mechanical knowledge. or,,, it could be an oil seal on the crankshaft, much more complicated.
these are not big, complicated jobs but get someone who has a bit of knowledge to assist so your son will learn how bits fit and what to do.
and also does it need a new, properly 'gapped' spark plug?
hope this helped a bit.