sounds like it should be looked at by a professional.
if your lucky the flywheel and stator jammed toghether
which is the parts behind the plastic cover by the foot shifter.
or it is the crank shaft piston may have locked up.
if you and son are up for a challenge we are here to guide you through it.
there is many diagnose procedures we can guide you threw if you choose to inspect it
basically it needs to be taken apart to inspect at different places lol